Interior angles of triangles always add up to equal 180 degrees, no matter the triangle. So, you can set all these expressions equal to 180 and solve algebraically for x.
180 = (4x + 28) + (x + 19) + (3x + 13)
180 = 4x + 28 + x + 19 + 3x + 13
180 = 8x + 60
120 = 8x
15 = x
Answer:
x = 15
To solve for angle measures, plug 15 for x into each expression.
4x + 28
4(15) + 28
60 + 28
88°
And so on.