The formula is
[tex]A=\frac{(a+b)\cdot h}{2}[/tex]To solve for b, first, we multiply the equation by 2
[tex]\begin{gathered} 2A=2\cdot\frac{(a+b)\cdot h}{2} \\ 2a=(a+b)\cdot h \end{gathered}[/tex]Then, we divide the equation by h
[tex]\begin{gathered} \frac{2A}{h}=\frac{(a+b)h}{h} \\ \frac{2A}{h}=a+b \end{gathered}[/tex]At last, we subtract a from each side
[tex]\frac{2A}{h}-a=a-a+b[/tex]