Manish's doubt

Algebra Level 2

Once Manish had a doubt. So he asked Rajdeep, the genius his doubt. Seeing the doubt, Rajdeep immediately took a pen and solved his doubt. Manish thanked Rajdeep for his kind help. Can you guess what was Rajdeep's answer if Manish's doubt was as follows:

$f(x)=a{ x }^{ 3 }+b{ x }^{ 2 }-4x-3,\quad where\quad a\quad and\quad b\quad are\quad constants$ Given that (x-1) is a factor of f(x)

Find the value of a+b