7. Two people are pushing on a \( 40 \mathrm{~kg} \). One is pashing with \( 250 \mathrm{~N} \) to the left and the other is pushing with a force of \( 180 \mathrm{~N} \) to the right.
a) If the box is not moving, what is the frictional force?
b) What is the coefficient of friction?