Respuesta :

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

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