Factoring a polynomial/simplifying

I have an test tomorrow. I studied everything..but i know my teacher will give us hard questions he did not even solve in the class..

how to factor this polynomial:

**4y^2 - 81**

i know that a^2 - b^2 = (a-b) (a+b)

Simplify:

**(m^2 n ^3)^-2 (m^-3 n^2)^-3**

we learned that if we have (m^2 m^3)^2 then the answer is m^2 n^3 x m^2 n^3 but how about if we have a -ve square or -3?

Simplify a radical:

**√8 + √18**

according to my teacher the solution is..

= √(2x2x2) + √(3x3x3)

=2√2 + 3√2

=5√3

how is that possible? :confused:

Please help

Thanks in advance :)