Answer:
f = xy²-2x² satisfies that F = ∇f.
Step-by-step explanation:
F(x,y) = (y² - 4x) i + 2xy j
We want f(x,y) such that

Lets find a primitive of y²-4x respect to the variable x. We need to think y (and y²) as constants here, so a primitive of y² would be xy² the same way that a primitive of k is xk (because we treat y² as constant). A primitive of x is x²/2, thus a primitive of 4x is 2x². Thus, a primitive of y²-4x is xy² - 2x². We can obtain any other primitive by summing a constant, however since we treated y as constant, then we have that

where c(y) only depends on y (thus, it is constant repsect with x).
We will derivate the expression in terms of y to obtain information about c(y)

Thus,
is constant. We can take f(x,y) = xy²-2x². This function f satisfies that F = ∇f.