Question 78526
{{{x*sqrt(18)-3*sqrt(8*x^2)}}}

First, simplify what is under the sqrt signs so you can "pull stuff out"...
you can re-write this as:

{{{x*sqrt(2*3*3)-3*sqrt(2*2*2*x*x)}}}

Now you can see that this can be re-written as:

{{{3x*sqrt(2)-6x*sqrt(2)}}}

This can be simplifed to be:
{{{-3x*sqrt(2)}}}