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
Any real number is a solution to one of the two variables of the equation, there is no unique solution.
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.