Answer: Yes it is a function
We can solve x-y = 5 for y to get y = x-5. This is a linear function. It graphs a straight line that passes the vertical line test.
The vertical line test is where you try to draw a single vertical line through more than one point on the function curve. In this case, we cannot draw such a vertical line, so the graph passes the vertical line test.
Plugging any input x into y = x-5 leads to exactly one y output value. This is another way to see we have a function.