(m+2)(m-2)

** You should learn to recognise this.

When you factorise a difference of 2 squares this is what you get. One bracket is the conjugate of the other.  Only the sign in the middle is different.

$$x^2-a^2=(x-a)(x+a)$$     

OR if you simplify

$$(x-a)(x+a)=x^2-a^2$$

Yours was 

(m-2)(m+2) so the answer should become obvious to you as   $$m^2-2^2=m^2-4$$

I guess you just want this to be expanded?

(m+2)(m-2) = m² - 2m + 2m - 2² = m² - 2² = m² - 4