Question

Determine the cutting force f exerted on the rod s in terms of the forces p applied to the handles of the heavy-duty cutter.

Answer 1

CASE 1: Direction vertically downward on the blade 

Assume pin force at E acts downward on the blade and CW is a positive moment 
Ey[3.4] - F[1.7] = 0 
Ey = F/2 

Case 2: Direction vertically upward on the blade . Assume CW is positive moment 

Ey[1.5sin19] – P[21 – 1.5sin19] = 0 
(F/2)[1.5sin19] = P[21 – 1.5sin19] 
F = 2P[21 – 1.5sin19] / [1.5sin19] 
F = 84P 

Answer 2

If you use the next formula with the data given in the exercise you are asking:
Ey[3.4] - F[1.7] = 0 
Ey = F/2 
and after that what you need to do is sum the moments of the handle about D to zero asumming it is a positive moment and we proceed like this
Ey[1.5sin19] – P[21 – 1.5sin19] = 0 
(F/2)[1.5sin19] = P[21 – 1.5sin19] 
F = 2P[21 – 1.5sin19] / [1.5sin19] 
F = 84P