how do you factorise x squared + 4x - 21

$$x^2+4x-21$$

 

You need to find 2 numbers that multiply to -21  (note: one will have to be neg and the other positive)

There aren't many to choose from   3,7       21,1

now these two numbers also have to add to +4 for the biggest one will be the positive one.

Easy  +7,-3

so the answer it

(x+7)(x-3)