1. You are dealing with two right triangles whose acute angle have the value $\displaystyle \theta$ and whose hypotenuses are $\displaystyle \overline{AD}$ and $\displaystyle \overline{AB}$(see attachment)
2. According to the sketch:
$\displaystyle x = 1.5 \cdot \cos(\theta)$
$\displaystyle y = 3.5 \cdot \sin(\theta)$
and: $\displaystyle x+y=2.5$
3. Plug in the terms of x and y into the last equation and afterwards multiply through by 2.