Greg had $17 more than fred, and together they had just enough money to by their mom a present that costs $85. how much money did greg have?

Define x:

Let Fred had x dollar.

Fred = x 
Greg = x + 17

Solve x :

x + x + 17 = 85
2x + 17 = 85
2x = 68
x = 34

Find the amount each of them had:

Fred = x = $34
Greg = x + 17 = 34 + 17 = $51

Greg had $51

Greg had $51 while Fred had $34.
Greg has $17 more than Fred: 51-34 = 17
Together, their money is enough to get their mom a present that costs $85 ($51 + $34 = $85)

So Greg have $51.