Start by writing an equation
x - 1/7x = 18
since there is no number in front of the first x it is an implied 1. In order to subtract we need an equivalent fraction with a common denominator so we change 1 to 7/7.
Now we can subtract
7/7x - 1/7x = 18
6/7x = 18
divide each side by 6/7 (multiply 18 × the reciprocal)
x = 18/1 × 7/6
x = 126/6
x = 21