Question 666125
The key is to break the square root into a perfect square times another number.

{{{sqrt(50) = sqrt(2*25) = sqrt(2)*sqrt(25) = 5*sqrt(2)}}}
{{{sqrt(128) = sqrt(2*64) = sqrt(2)*sqrt(64) = 8 *sqrt(2)}}}
{{{2sqrt(50) - 3sqrt(128))}}}
{{{2*(5sqrt(2)) - 3*(8sqrt(2)) = 10sqrt(2) - 24sqrt(2) = highlight(-14sqrt(2))}}}