I do not understand how to solve this problem.

7 - 1/4 (j-8) =6

You are supposed to find out what value of j satisfies that equation.

You need to clarify if
1/4 (j-8) = (1/4)(j-8), or
1/4 (j-8) = 1/[4(j-8)]

In the first case,
(1/4)(j-8) = 1
j -8 = 4
j = 12