In order to solve for c we just have to remember one simple rule: what we do in one side of the equal "=" we should do it in the other side.
In this case:
2c + 7n + z = y
↓ substracting z both sides
2c + 7n + z - z = y - z
2c + 7n + 0 = y - z
2c + 7n = y - z
↓ substracting 7n both sides
2c + 7n - 7n = y - z - 7n
2c + 0 = y - z - 7n
2c = y - z - 7n
↓ dividing by 2 both sides
[tex]\begin{gathered} \frac{2c}{2}=\frac{y-z-7n}{2} \\ c=\frac{y-z-7n}{2} \end{gathered}[/tex]Then, we have the answer:
[tex]c=\frac{y-z-7n}{2}[/tex]