Given equation is
[tex]A = \frac{a+b+c+d}{4} [/tex]
Now we can multiply both sides by 4
[tex]4A = a+b+c+d[/tex]
On subtracting (a+b+d) on both sides.
[tex]4A -(a+b+d) = (a+b+c+d) - (a+b+d)[/tex]
[tex]4A - a - b - d = c[/tex]
So c is
[tex]c = 4A - a -b-d[/tex]