Say
Thus :
Expand :
Rearrange terms :
Factorize :
Make x the subject by dividing :
Thus the inverse function of is
Does it make sense ?
By the way, this trick can help you find answers easily so as to be able to solve this type of questions quicker : can you spot some kind of relation between the numbers in the function and the numbers in the inverse function ?
EDIT : woah Raoh now this is "compressed" maths, now I realize how much of a waste of paper my writing style would be in an exam . I detailed carefully each step because Hunterage asked for it though .