If the equation is (3x) * (7x-9) would the answer be 21x-9?

Thank you i get it now!

No. If you are multiplying a number by a multi-term number (polynomial), you have to multiply that single number by every term in the polynomial:

a(x + y) = ax + ay

So your:

3x(7x - 9)

You multiply the 3x by the 7x and -9 to get:

21x² - 27x