Rearrange the terms and group the first two and the second two terms together:
15cd+ 12c - 20d - 16
From the first two terms take out a common factor of 3c, and from the second two terms take out a common factor of -4, and hope for the best--a common factor must result, or this doesn't work!!
15cd+ 12c - 20d - 16
3c(5d+ 4) -4(5d+ 4)
Glory Hallelujah!! We must be living right!! We DO have a common factor of (5d+4).
Take out the common factor of (5d+4):
(5d+4)(3c-4)
