($4x\mathrm{y)}dx+({\mathrm{x^}}^4+7{\mathrm{y^2}}^{})dy=0$

The solution says the equation is exact, but i dont understand how. The derivative of 4xy with respect to y is 4x. What is the derivative of (x^4+7y^2) with respect to x. The answer says 4x, but how is this correct? Derivative of x^4 is 4x^3, so the equation can’t be exact. Someone please explain who’s right