Simplifying
36r4 + 29r2 + -20

Reorder the terms:
-20 + 29r2 + 36r4

Factor a trinomial.
(-5 + -4r2)(4 + -9r2)

Factor a difference between two squares.
(-5 + -4r2)((2 + 3r)(2 + -3r))

Final result:
(-5 + -4r2)(2 + 3r)(2 + -3r)