because we know its an isosceles triangle angle A and C will both be the same and since we know A = 69 C will also be 69. we also know that angles in a triangle add to 180 so take away the two 69s from the 180 to get 42
then we work backwards if its 5x + 2 then we need to do 5x - 2 to get 40 then divide it by 5 to get your answer 8!
