Answer
$$25 .$$
Work Step by Step
We find $b$ by multiplying both sides by 2 as follows:
$$ A=\frac{1}{2}h(a+b)\Longrightarrow 2A=ha+hb \\\Longrightarrow hb=2A-ha.$$ By dividing both sides by $h$, we get $$ b=\frac{2A-ha}{h}=\frac{2A}{h}-a. $$ By substituting, we have $$ b=\frac{2*260}{13}-15=40-15=25 .$$
