The first step in simplifying this expression is to identify which, if any, terms can be combined. Remember, only like terms — constant terms or terms with the same variable and the same exponent — can be combined.
a^2 - 4a - 4a + 16
In this case, we have one a^2-term, two a-terms, and one constant.
Only the a-terms can be combined by factoring out a and then subtracting the coefficients.
