how do I find the value of variable and the measure of the angles

1) Since the sum of interior angles yields 180º, no matter which type of triangle it is.
2) Then we can write
3x+12 +x +6x +8 = 180
10x +20 = 180º
10x = 180-20
10x = 160
x= 16
2.2 Let's find out the measurement of each angle
m∠ A= x Plug into that the value of x
m∠ A = 16º
m∠B= 3x+12 Plug into that the value of x
m∠B = 3(16) +12
m∠B = 48+12
m∠B =60
3) Then the answers are:
x = 16
m∠ A = 16º
m∠B =60