Lines $l$ and $m$ are parallel and cut by transversal $n$. The angles $(3x + 5)°$ and $(x + 15)°$ are co-interior angles (also called same-side interior or consecutive interior angles).

Co-interior angles formed by parallel lines and a transversal are supplementary (sum to $180°$):

$(3x + 5) + (x + 15) = 180$

$4x + 20 = 180$

$4x = 160$

$x = 40$