#### Answer

$(A-B)^2$

#### Work Step by Step

RECALL:
A perfect square trinomial can be factored using either of the following formulas:
(i) $a^2-2ab+b^2=(a-b)^2$
(ii) $a^2+2ab+b^2=(a+b)^2$
The given perfect square trinomial is similar in form to the one in formula (i) above.
Thus, the missing expression in the given statement is:
$(A-B)^2$