When bella pushes a box with a mass of 5.25 kilograms with a force of 15.75 newtons, it accelerates at a rate of 2.5 meters/second2. what is the force due to friction?

i have no clue whats going on. can anybody me

Sure !

Start with Newton's second law of motion:

                     Net Force = (mass) x (acceleration) .

This formula is so useful, and so easy, that you really
should memorize it.

Now, watch:

The mass of the box is 5.25 kilograms, and the box is
accelerating at the rate of  2.5 m/s² .
What's the net force on the box ?

                    Net Force = (mass) x (acceleration)

                                     = (5.25 kilograms) x (2.5 m/s²)

                     Net force =       13.125 newtons .

But hold up, hee haw, whoa !  Wait a second !
Bella is pushing with a force of 15.75 newtons, but the box
is accelerating as if the force on it is only 13.125 newtons.
What happened to the rest of Bella's force ? ?

==>  Friction is pushing the box in the opposite direction,
and cancelling some of Bella's force.

How much ?

            (Bella's 15.75 newtons) minus (13.125 that the box feels)

           =      2.625 newtons backwards, applied by friction.