Answer
$\theta = \pi - arctan(\frac{75}{100-x}) - arctan(\frac{150}{x})$
Work Step by Step
Let $~A~$ be the angle between the ground and the short building.
$tan~A = \frac{75}{100-x}$
$A = arctan(\frac{75}{100-x})$
Let $~B~$ be the angle between the ground and the tall building.
$tan~B = \frac{150}{x}$
$B = arctan(\frac{150}{x})$
We can find an expression for $\theta$:
$A+\theta+B = \pi$
$\theta = \pi - A - B$
$\theta = \pi - arctan(\frac{75}{100-x}) - arctan(\frac{150}{x})$
