Angle ECD and Angle FDC are same-side interior angles. They lie on the same side of the transversal and are on the inside of the two parallel lines. Same-side interior angles are supplementary, meaning that they add up to 180.
(40x + 15) + (5x + 30) = 180
45x + 45 = 180
45x = 135
x = 3
Angle ECD = 40(3) + 15 = 120 + 15 = 135
Angle FDC = 5(3) + 30 = 15 + 30 = 45
Hope this helps!