How do I solve this math question?

Is the -7/9 the coefficient of w then?
so it's:

21 = (-7/9)w

If so, you need to multiply both halves by the reciprocal of the coefficent, which is -9/7, so we have:

21(-9/7) = w
-27 = w

21 = -7/9w
i.e 21*9 = -7/9w*9
i.e 189 = -7w
i.e -189/7 = w
i.e -27 = w
i.e w = -27

Tip #1: If you multiply a fraction by the denominator (the bottom part) you get rid of the bottom part. For example, 1/2 * 2 = 1 (you just got rid of the bottom 2).

Tip #2: If you do the same thing to BOTH SIDES of any equation then the equality remains. For example, 4 = 2 + 2 stays an equality if I add 10 to both sides: 5 + 10 = 2 + 2 + 10, right?

So use tip #1 and tip #2 to solve your equation.

21 * 9 = -7/9w * 9 (tip #2)
21 * 9 = -7w (tip #1)
189 = -7w
w = -27

21 = -7/9W
21 * 9 =-7W
(21/7)*9=-W
(3*9)/-1 = W
-27 =W

multiply 9 to both sides

189=-7w

divide -7 to both sides

-27= w

w=-27

21 = 7/9w
7/9w = 21
7w = 21/9 =7/3
w = 7/3/7 = 1/3

21 = -7/9(w)
21 = -7w/9
9(21) = -7w
w = 189/(-7)
w = -27

w= -27

21 = -7/9 w

Divide (-7/9) from both sides. Giving:
21 ÷ (-7/9) = w

When dividing two fractions we must reciprocate (flip over) the fraction and multiply. Thus:
21 × (-9/7) = w

Do this using a calculator (or mentally). This gives:
-27 = w

Hope this helps :D