First let's call the vertex between D and C E and the one between A and D (next to x) F.
And since the sum of angles in a triangle is constant (equal to 180°) it follows that the third angles are also equal that is
Now since all three angles are equal it follows that the triangles CDE and AFD are similar, hence their sides must be proportional so we have
Since ABCE is parallelogram `|AB|=|CD|=>|AB|=7.5` also `|AB|=|DC|=>|DC|=5`
So we have
`x=6` <-- Your solution