Answer
$A$ varies jointly as $b$ and $h$.
Work Step by Step
It means that if we multiply $b$ by a factor $m$ and $h$ by a factor $n$, then $A$ will be multiplied by a factor $mn$.
$A_1=\frac{1}{2}b_1h_1$
Now suppose that $b_2=mb_1$ and $h_2=nh_1$
$A_2=\frac{1}{2}b_2h_2=\frac{1}{2}mb_1nh_1=mn\frac{1}{2}b_1h_1=mnA_1$
$A_2=mnA_1$
