The box is in equilibrium, so Newton's second law says
<em>n</em> + (-<em>w</em>) = 0
65 N + (-<em>f</em> ) = 0
where <em>n</em> denotes the magnitude of the normal force, <em>w</em> denotes the weight of the box, and <em>f</em> denotes the magnitude of the friction force.
The box has a weight of
<em>w</em> = (25 kg) (9.80 m/s²) = 245 N
so <em>n</em> = 245 N, too.
The friction force has magnitude
<em>f</em> = 65 N
and is proportional to the normal force by a factor of <em>µ</em>, the coefficient of kinetic friction. So we have
65 N = <em>µ</em> (245 N) → <em>µ</em> ≈ 0.26
