Approximately what force, fm must the extensor muscle in the upper arm exert on the lower arm to hold a 7.6kg shot put? assume the lower arm has a mass of 2.8kg and it's cg is 11.6cm froom the pivot point.

Lets do the sum of the forces about the elbow joint.

Fm = Force of Muscle; Fe = Force Elbow; Fb = Force Ball

Sum Force about Joint = (-2.5)Fm + 12.5Fe + 30Fb = 0

(-2.5)Fm + 12.5(2.8) + 30(6.9) = 0

Fm = 96.8kg

Fm = 96.8 * 9.8 = 948.6N

Do you understand why the -2.5 is negative?
Because I put the origin at the joint. So when you go left it is negative and when you go right it is positive.