2) 10sqrt(c) - 20sqrt(c) + 30sqrt(c) = 20sqrt(c)
3) 3sqrt(5) + 4sqrt(5) - 10sqrt(5) = -3sqrt(5)
4) 2sqrt(2) + 5sqrt(2) - 3sqrt(2) = 4sqrt(2)
5) sqrt(25*6p) - sqrt(9*6p) + sqrt(4*6p) = 5sqrt(6p) - 3sqrt(6p) + 2sqrt(6p) = 4sqrt(6p)
6) sqrt(36*11m) + 2sqrt(4*11m) - 15sqrt(16*11m) =
= 6sqrt(11m) + 2*2sqrt(11m) - 15*4sqrt(11m) = -50sqrt(11m)