Please explain how to find the formula for gradient on the problem below

Perform a reduced gradient search for the problem :

F_total = F₁ + F₂ + F₃

When

F₁(P₁)= 561 + 7.92(P₁)+0.00156(P₁)²

F₂(P₂)= 310+ 7.85(P₂)+0.00194(P₂)²

F₃(P₃)= 78 + 7.97(P₃)+0.00482(P₃)²

With constraint

P₁+P₂+P₃=850

And initial conditions P1= 300 MW, P2 = 200 MW and a step size of a = 10