(6-sqrt12)(6+sqrt12)
FOIL: 36 + 6sqrt(12) - 6sqrt(12) - 12 = 36 - 12 = 24.
(3+sqrt8)(3-sqrt8)
FOIL: 9 + 3sqrt(8) - 3sqrt(8) - 8 = 9 - 8 = 1.
Note: It may be easier to notice that both of these are a "difference of squares:"
(6-sqrt12)(6+sqrt12) is in the format (a - b)(a + b) = (a^2 - b^2) since the two middle terms [ab & -ab] cancel out.
(a^2 - b^2) --> (6^2 - (sqrt(12)^2)) --> (36 - 12) = 24.
Same for the second one --> (3^2 - (sqrt(8))^2) = (9 - 8) = 1.