Answer
$(r-3s)^2$
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 trinomial can be written as:
$=r^2-2(r)(3s)+(3s)^2$
The trinomial is a perfect square.
Factor using formula (i) above with $A=r$ and $B=3s$ to obtain:
$=(r-3s)^2$
