Suppose x-6 is listed as a possible answer. That is zero if x = 6. So put x=6 into the original x^2 + 4x - 60 to get 6^2 + 4*6 - 60 = 36 + 24 -60 = 0. hence x-6 is a factor.
Answer (x-6)
on this hand :x-5 is not a factor, because plugging x=5 into does not work.