The sum of the angles of a triangle is 180°. So you can do:
91° + (8x + 3)° + (10x - 4)° = 180° To find x, you need to isolate/get the variable "x" by itself in the equation. First combine like terms (terms that have the same variable and power/exponent]
91 + 3 - 4 + 8x + 10x = 180 (I rearranged for the like terms to be next to each other)
90 + 18x = 180 Subtract 90 on both sides
18x = 90 Divide 18 on both sides to get "x" by itself
x = 5
PROOF
91° + (8x + 3)° + (10x - 4)° = 180° Plug in 5 for "x"
91° + (8(5) + 3)° + (10(5) - 4)° = 180°
91° + (40 + 3)° + (50 - 4)° = 180°
91° + 43° + 46° = 180°
180° = 180°