#### Answer

b=39

#### Work Step by Step

The variable appears twice in the numerator, 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 $6b+18=7b-21$. We subtract by 6b on both sides to get that $18=b-21$. We cancel out subtraction by adding 21, isolating b and getting the answer: b=39.