Let,

AB, CD and EF intersect at O

∠COB = ∠AOD (Vertically opposite angle)

∠AOD = 3x + 10 (i)

∠AOE + ∠AOD + ∠DOF = 180^o (Linear pair)

x + 3x + 10^o + 90^o = 180^o

4x + 100^o = 180^o

4x = 80^o

x = 20^o