We get the variable c on one side of the equation and everything on the other side of c.
2c + 5(c - 5) = 10 ; Start
2c + 5c - 25 = 10 ; Distribute the 5 across c and -5
7c - 25 = 10 ; Combine the like terms 2c and 5c
7c = 35 ; Add 25 to each side of the equation
c = 5 ; Divide both sides by the coefficient of c. Which is 7