Area of the trapezoid is $A=\dfrac{1}{2}h(U+L)$ U is the upper base and $U=2x+10$ & lower base is $L=10$
Because the bases of a trapezoid are parallel the upper angle is $\theta$, alternate angles.
Using trigonometry we get $\cos(\theta)=\frac{x}{10}~\&~\sin(\theta)=\frac{h }{10} $
Now we are able to expresss area $A(\theta)$ and maximize using $d\theta$ .