Answer
The force on the forearm from the triceps muscle is directed upward.
Work Step by Step
Note that the tension $T$ in the rope is equal to the weight of the block.
We can use the sum of the torques about the rotation axis to find the torque $\tau_t$ due to the force $F$ from the triceps muscle:
$\sum \tau_i = 0$
$\tau_t - (15~cm)(2.0~kg)(9.8~m/s^2)~sin~120^{\circ}+(35~cm)(15~kg)(9.8~m/s^2)~sin~60^{\circ} = 0$
$\tau_t = (15~cm)(2.0~kg)(9.8~m/s^2)~sin~120^{\circ}-(35~cm)(15~kg)(9.8~m/s^2)~sin~60^{\circ}$
$\tau_t = -4201~N\cdot cm$
The negative torque due to the force from the triceps muscle shows that the torque is clockwise. By the right hand rule, the force on the forearm from the triceps muscle must be directed upward.
