A = \frac{1}{2} bh \\ \\ 2A =bh \\ \\ \purple{b = \frac{2A}{h}} \\ \\ plug \: A = 12 \: and \: h = 4 \\ \\ b = \frac{2 \times \cancel{12}}{ \cancel4} \\ \\ b = 2 \times 3 \\ \\ \\ b \: = 6 \: in
A = \frac{1}{2} bh \\  \\ 2A =bh \\  \\  \purple{b =  \frac{2A}{h}}  \\  \\ plug \: A = 12 \: and \: h = 4 \\  \\ b =  \frac{2 \times \cancel{12}}{ \cancel4}  \\  \\ b = 2 \times 3 \\  \\ b \:  = 6  \: in