Here it is:
The first set has (0,1) and (0,5), which assigns to the *same* x, more than one value of *y*. That's not a function. It's a curve. Think about a circle, or instance. It's NOT a function, but it's a curve.
The second one has for each *x* a single *y*
BTW, there's no problem if for *different* x there is the same *y*:
y = 5,
all x, have the same y, equal to 5. That's a function, or y = x^2, etc
See, please, the offered decision, note, that 2x^2yxy^2 is as <span>2x^2y+xy^2.</span>