How do you solve for b in a=3/4bh?

It is $b = \frac{4 \cdot a}{3 \cdot h}$

Explanation:

First step multiply both sides with $4$ hence you get

$4 \cdot a = 3 \cdot b \cdot h$

Divide both sides with $3 \cdot h$ hence you get

$4 \cdot \frac{a}{3 \cdot h} = b$

or $b = \frac{4 \cdot a}{3 \cdot h}$