Answer:
x = 20
Step-by-step explanation:
The triangle has 2 congruent sides and is therefore isosceles with 2 congruent base angles, both 3x + 8
5x + 12 and 3x + 8 lie on a straight line and sum to 180° , that is
5x + 12 + 3x + 8 = 180
8x + 20 = 180 ( subtract 20 from both sides )
8x = 160 ( divide both sides by 8 )
x = 20
