frac{21b^x}{nb^3}=7b^9
21b^x
_____ = 7b^9
n b^3
This implies that
21 / n = 7 ⇒ n = 3
And
b^x / b^3 = b^9 means that
b^x = b^9 * b^3
b^x = b^12
x = 12