Find the general solution to the differential equation y''' - 3y'' - 15y'.
a) y = C₁e (5x) + C₂e (-3x) + C₃e (0x)
b) y = C₁e (5x) + C₂e (-3x) + C₃x
c) y = C₁e (-5x) + C₂e (3x) + C₃e (0x)
d) y = C₁e (-5x) + C₂e (3x) + C₃x