about a month ago we were set some homework (so no worries it's overdue now 8) )

the question was

if y=ax^3 + bx^2 + cx

when dy/dx=15 and x=2

what is a, b and c

now I still havn't figured it out (forgot to be honest ) BUT I'm not looking for a simple answer to the problem

the only way I can think of to solve this is using trial and improvement, which is a problem because even assuming a,b,c are not over 5 (which could be wrong) there are still alot of posibilities

So is there some kind of method or formula to use to work this out? or should I just continue guessing?