Solve P=a+2b+3c for b

What you do here is, isolate everything to solve for the variable b. Steps as following:

P-3c=a+2b

P-3c-a=2b

(P-3c-a) / 2=b

Final answer: (P-3c-a) / 2 = b

If you're solving for b, that means you want to isolate b to one side.

P = a + 2b + 3c

Subtract 3c from both sides:

P - 3c = a + 2b

Subtract a from both sides:

P - 3c - a = 2b

Divide both sides by 2:

b = (P - 3c - a) / 2