They both are complementary angles i.e their sum must equals 90°
4x+ 31 + 6x + 39 = 90
10x + 70 = 90
10x = 90-70
10x = 20
x = 2
we have to find angle ADC
just put the value of x in that expression for that angle.
angle ADC = 6x + 39 = 6 (2)+39 = 12 + 39 = 51°