You cannot simply solve this equation; there is no unique value for x and y. What you can do is solve for x in terms of y or y in terms of x.
x in terms of y is x = 17.5-y
y in terms of x is y = 17.5-x
We can only give you the solution to one variable when the other is given, for example:
When x = 0
y = 17.5
When x = 1
y = 16.5
So, we cannot give you a fixed solution to this problem.