Question 423846
In general, the solution (using FOIL) would be: 
{{{(a*sqrt(b) + c*sqrt(f))(a*sqrt(b) - c*sqrt(f)) = a^2b - c^2f}}}, since the middle two terms cancel out.
Using an example, {{{(5*sqrt(4) + 2*sqrt(9))(5*sqrt(4) - 2*sqrt(9))}}}
Solving gives 25*4 - 4*9 = 100 - 36 = 64