To multiply these two terms you multiply each individual term in the left parenthesis by each individual term in the right parenthesis.
#(color(red)(5sqrt(2)) + color(red)(3))(color(blue)(sqrt(2)) - color(blue)(2sqrt(3)))# becomes:
#(color(red)(5sqrt(2)) xx color(blue)(sqrt(2))) - (color(red)(5sqrt(2)) xx color(blue)(2sqrt(3)))) + (color(red)(3) xx color(blue)(sqrt(2))) - (color(red)(3) xx color(blue)(2sqrt(3))))#
#(5 * 4) - (10sqrt(2)sqrt(3)) + 3sqrt(2) - 6sqrt(3)#
#20 - (10sqrt(2)sqrt(3)) + 3sqrt(2) - 6sqrt(3)#
We can use this rule of radicals to complete the simplification:
#sqrt(color(red)(a)) * sqrt(color(blue)(b)) = sqrt(color(red)(a) * color(blue)(b))#
#20 - (10sqrt(color(red)(2))sqrt(color(blue)(3))) + 3sqrt(2) - 6sqrt(3)#
#20 - (10sqrt(color(red)(2)*color(blue)(3))) + 3sqrt(2) - 6sqrt(3)#
#20 - 10sqrt(6) + 3sqrt(2) - 6sqrt(3)#