Answer:
a = P - b - c
Explanation:
given
P = a + b + c ( to isolate a we require to subtract b and c from both sides )
subtract b from both sides
P - b = a + b - b + c ( simplify )
P - b = a + c ( subtract c from both sides )
P - b - c = a + c - c , that is
P - b - c = a