(2x + 7) ( x - 15) = -43 + jx
2x^2 - 23x - 105 = -43 + jx
2x^2 - (23+ j)x - 62 = 0
If this has one real root, then the discriminant = 0 .......so......
(23 + j)^2 - 4 (2) (-62) = 0
(23 + j)^2 + 496 = 0
(23 + j)^2 = -496 take both roots
23 + j = 4sqrt (31) i 23 + j = -4 sqrt (31) i
j = 4sqrt (31) i - 23 j = -4sqrt (31) - 23