Fill in the blank for remainder therom

x 3 + x 2 - 21x - 45 = (x + 3)(x - 5)(x+? )

Not sure about remainder theorem but I'm sure that the last terms should all multiply tho the last term

see the expanded form is -45

so the last terms of each binomial should multiply to -45

3 times -5 times ?=-45
-15 times ?=-45
divide by -15
?=3

the question mark is 3

(3x+2)(2x+5) , (x-3)(x-1), (2x-3)(5x+1), (3x-1)^2? (3x+2)(2x+5) ,
(x-3)(x-1),
(2x-3)(5x+1),
(3x-1)^2