Factor -5a^2+10ab^2-20+25a^3b
i need !

Here is the correct form to start factoring from:
25a^3 - 5a^2 + 10ab^2 - 20

ok...
divide all the terms by 5 and you should get:
5a^3 - a^2 + 2ab^2 - 4

First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below: multiply exponents per exponent rules to get: remember you subtract exponents from the denominator and add exponents in the numerator so: simplify further: so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.