A function is a special relation whose every input corresponds with a single output. To be a function, each input is allowed to pair with only one output element.
[tex] x^2+y-15=0\\ Adding \; 15 \; subtracting \; x^2\; on\; both\; sides\\ \\ y=-x^2+15 [/tex]
For a parabola equation like above, there is no restriction in the domain. For every x value there is a unique y value.
Conclusion:
The equation [tex] x^2+y-15=0 [/tex] is BOTH a function and a relation!
