`a) M = 4sqrt{x^2 + 1} - 2sqrt{16(x^2 + 1)} + 5sqrt{25(x^2 + 1)}`
`= 4sqrt{x^2 + 1} - 8sqrt{x^2 + 1} + 25sqrt{x^2 + 1}`
`= 21sqrt{x^2 + 1}`
`b) N = 2/(x + y)sqrt{(3(x + y)^2)/4} with x + y > 0`
`= 2/(x + y)(sqrt3(x + y))/2`
`= sqrt3`
`c) P = 3/(3a - 1)sqrt{5a(1 - 6x + 9a^2)} with a > 1/3`
`= 3/(3a - 1)sqrt{5a(1 - 3a)^2}`
`= 3/(3a - 1)sqrt{5a}|1 - 3a|`
`= 3/(3a - 1)sqrt{5a}. (3a - 1)`
`= 3sqrt{5a}`