do you know fractions?:(?

Multiply both sides by -1/20. Remember, you can multiply the equation by anything you want, as long as you do the same operation to both sides. -1/20 was picked because it's the inverse of the coefficient (the number before the variable) of 'a'. The inverse is just a number over itself, and you use them to cancel out terms. The idea being that if we can cancel out the coefficient, we'll have the variable (a) isolated and that will tell us its value.

The right side becomes (-20a) * (-1/20). A negative times a negative is a positive, so it's (20a) * (1/20). 20 times 1/20 is the same as 20 / 20. Any number divided by itself is 1. So the 20's cancel out (see why we picked -1/20 now?). That leaves us with just the variable 'a' on the right side.

The left side then becomes (-18)*(-1/20). Two negatives are a positive, so it's now 18*(1/20). You can rewrite that as 18/20. 18/20 can be simplified by dividing both numerator and denominator by 2, their common factor. So 18/2 = 9, and 20/2 = 10. Now the left side becomes 9/10.

Put the left and the right back together, and we have:

9/10 = a

And there's your answer, using multiplication.

To check your answer, replace the 'a' in the original equation with your answer, and make sure the equation is still true.

So substituting our answer for 'a' into the equation, we have:

-18 = -20 * 9 / 10 (the 9 / 10 is our value for a, remember)

If you multiply out the right side, it becomes:

-20 times 9 is -180.
-180 divided by 10 is -18

So we have: -18 = -18, which is true. So our solution for 'a' is correct!

I hope that helps!

- 18 = - 20a
- 9 = - 10a
a = - 9/- 10
a = 9/10

Answer: a = 9/10

Proof:
- 18 = - 20(9/10)
- 18 = (- 20/10)(9/[10/10])
- 18 = - 2(9)
- 18 = - 18

18 = 20 a
9 = 10 a
a = 9 / 10

first you must isolate the variable a:

-18 = -20a

Multiply both sides of the equation by 1/20:

1/20 * -18 = 1/20 * -20a

-18/20 = -20a/20

Reduce:

-9/10 = -a

Multiply by -1:

a = 9/10

Plug in for a: -18 = -20 * (9/10)

-18 = -180/10
-18 = -18

I guess you could multiply both sides of this equation by -1/20.

This would give you
-18/(-20) = -20a/(-20)
9/10 = a

Check by substituting 9/10 for a in the original equation:

-18 = -20*(9/10)?
-18 = -180/10? Yes! it checks.

-18 = -20a
a= -18/-20
a = -.9

To check, just multiply -.9 (which used to be the letter a) by -20

(-20)(-.9) = -18

-18 = -20a
20a = 18
a = 18/20
a = (18/2)/(20/2)
a = 9/10 (0.9)