4a^2 - 28a + 45
try using factor by grouping
start with 4(45) = 180 then find the factors of 180 that add to be -28
factors of 180 are: 1, 180 2, 90 3, 60 4, 45 5, 36 6, 30
9,20 10, 18 (notice that 10 + 18 = 28, so if we use them both as negatives, we can get -28)
now write
4a^2 -10a - 18a + 45
group them (4a^2 - 10a) -(18a - 45)
find the greatest common factor of each pair
2a(2a - 5) - 9(2a - 5) notice they both have 2a - 5 in common
now "undistribute" (in other words put the 2a and -9 together
(2a-9)(2a-5)
you are now factored