6X − 7[X − (2 − X)]
First multiply out the square brackets( X=the letter 'x', and x=multiplication):
6X − 7[X − (2 − X)] = 6X - 7X - (-7)x(2 - X)
(be careful here with the negative signs, it's easy to lose a sign or to get confused, so it's best to write it out with as many steps as you want)
= -X + 7(2 - X)
Then multiply out these brackets:
= -X + 7x2 + 7x(-X)
= -X + 14 -7X
= -8X +14