Supplementary = 180 degrees...
A + B = 180
3x + 12 + 7x + 38 = 180
10x + 50 = 180
10 x = 130
x = 13
To find the A just substitute x in...
3 ( 13) + 12
A = 51
To check substitute the x..
A + B = 180
3(13) + 12 + 7(13) + 38 = 180
51 + 129 = 180
180 = 180
