To solve for c, subtract all the terms on the right side not containing c.
P -(a + b + (d/4)) = c
c = P - a - b - (d/4)
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In such equations, parentheses are commonly missing. If that is the case here, then first we undo the division by 4, then we subtract the "not c" terms.
P = (a + b + c + d)/4 . . . . maybe what you meant
