Solve a= (a + b + c + d)/4 for c

[math_celebrity]

Solve a= (a + b + c + d)/4 for c

Cross multiply:
4a = a + b + c + d

Subtract a + b+ d from each side to isolate c:
4a - a - b - d = a + b + c + d - a - b - d

Canceling the a, b, and d from the right side, we get:
c = 3a - b - d