Assuming that a, b, and c are the angle of a triangle.
180° = a + b + c.
This is because the sum of all the angles in a triangle is 180°.
Now we plug in the value of the variables.
180° = 5x + 4x + x
180° = 10x
18° = x
Now that we know what x is we can plug it in:
a = 5(18°) = 90°
b = 4(18°) = 72°
c = (18°) = 18°