x = amount of money he had in the beginning
Because he spent 3/7, he has 4/7 left.
(4/7x)
We have to take 1/4 off THIS value. He has 3/4 of this value left.
(4/7x)1/4
Now, lets set it up as an equation
(4/7x)1/4 = 120
Multiply both sides by 4
4/7x = 480
Multiply both sides by 7/4
x = 480 * 7/4
x = 840
Hope his helped!