-j + 5 = j - 7
add j to both sides
-j + j + 5 = j + j - 7
the j's on the left cancel out
5 = 2j - 7
add 7 to both sides
5 + 7 = 2j - 7 + 7
the j's on the right cancel out
12 = 2j
divide both sides by 2
12/2 = 2j/2
the 2's on the right cancel out
6 = j
Answer: j = 6