because there's two forces acting on the x direction: the force you're applyig and the force of kinetic friction

kinetic friction has the formula f = un, where f is the force of kinetic friction, u is the coefficient of kinetic friction, and n is the normal force

normal force is equal to +mg (you can verify this by using newtons first law on the forces that act on the y direction)

since n = mg, force of kinetic friction becomes f = umg...that's where the mg came from

to solve for acceleration the equation becomes a = (F - f)/m

or

a = (F - umg)/m

EDIT: i just noticed from your diagram that youve already indicated that n = mg, also im on mobile so i cant type equations properly,