From the diagram, angles x+58 and x+128 are supplementary, this means that their addition is equal to 180 degrees, that is,
x + 58 + x + 128 = 180
(x + x) + (58 + 128) = 180
2x + 186 = 180
2x = 180 - 186
2x = -6
x = (-6)/2
x = -3
