Work Step by Step
The variable appears in the denominator, so we know we should use the Cross Products Property. Recall, cross multiplying refers to when we multiply the numerator of the first fraction by the denominator of the second fraction and set that equal to the numerator of the second fraction times the denominator of the first fraction. We first cross multiply to get that $21b=3b-12$. We now subtract by 3b on both sides to get that $18b=-12$. We then divide by 18 on both sides to get that $b=-12/18$, which equals -2/3.