Verified answer

5x² + 10x + √(x² + 2x - 15) = 123

5(x² + 2x - 15) + 75 + √(x² + 2x - 15) = 123

√(x² + 2x - 15) = t >=0

5t² + t - 48 = 0

D = 1 + 4*5*48 = 961 = 31²

t12 = (-1 +- 31)/10 = -32/10   3

t = -32/10 <0 нет

t = 3

√(x² + 2x - 15) = 3

x² + 2x - 15 = 9

x² + 2x - 24 = 0

D = 4 + 96 = 100

x12 = (-2 +- 10)/2 = -6  4

ответ x = {-6, 4}