Question

How do i solve this im totally stuck

Answer 1

The answer is 6 sqrt(5).

When adding or subtracting radicals with coefficients you simply add/subtract the coefficients. If you are familiar with functions with variables, set x = sqrt(5) then we have

18x - 12x

Now it looks like something you are familiar with and it seems like an obvious answer

6x

now replace the x with the sqrt(5) and you get

6sqrt(5)