I assume you are looking for a particular solution for the differential equation:
dy/dx = 12x2 + 3x-5
∫dy = ∫(12x2 + 3x-5)dx
y = 6x3 + (3/2)x2 - 5x + C
where y=15 when x = 2:
15 = 6(2)3 + (3/2)(2)2 - 5(2) + C
15 = 6(8) + (3/2)(4) - 10 + C
15 = 48 + 6 - 10 + C
15 = 32 + C
C = -17
Therefore the particular solution is:
y = 6x3 + (3/2)x2 - 5x - 17