Answer:
24
Step-by-step explanation:
Given two fractions
and 
Factor the denominators of both fractions:

You can see that two digits 2 are the same in both denominators and there are two more factors: 2 in number 8 and 3 in number 12.
Thus, the least common denominator for
and
is

The third one I think I’m sorry if I get this wrong
Minus 6 from both sides, 4x = 16, divide by 4, x = 4
2(3)^3 + 4(3)^2-6(3)
2x 27 + 4x9 - 18
54 + 36 -18
= 72