A triangle's angles always add up to 180, so you can add them together and set them equal to 180:
3x + 13 + x + x - 8 = 180
Combine like terms:
5x + 5 = 180
Now get the variable separate from the numbers by subtracting 5 from each side:
5x = 175
Divide by 5 to solve for x:
x = 35
Now plug in 35 for x to solve for angle A:
3(35) + 13 = 118
The measure of angle A is 118 degrees
Hope that helped
