Answer
(r+1)(r-4)
Work Step by Step
You are given $r^{2}$-3r-4. Factor it out:
$r^{2}$+r-4r-4 -break out the middle value-
($r^{2}$+r)+(-4r-4) -group the first two and last two terms-
r(r+1)-4(r+1) -Find the GCF of each grouping-
(r+1)(r-4) -factor out the common term-
=(r+1)(r-4)
