( X+ a)( X+b) = X^{2} + ( a+b)X+^{ }ab
This is a general formula, where Multiplying binomial by binomial result in a trinomial( polynomial of 2nd degree or quadratic).
You have to be able to factor any quadratic 2 two binomial, and solve it.
and expand it.
( X-1) ( X +30) = X^{2 }+ 29X - 30
( X - 5) ( X +6 ) = X^{2 }+ X -30
( X + 2) ( X -15) = X^{2} - 13X - 30 You see that Coeficient of X is ( a+b) and constant is ab