6 - 8c = -10 - 10c
and you're solving for c. what you really need to do in problems like this is combine like terms: -10 and 6 are like terms, because they are numbers without variables. -8c and -10c are like terms, because they are variables with coefficients.
6 - 8c = -10 - 10c ... add 10c to both sides
6 + 2c = -10 ... subtract 6 from both sides
2c = -16 ... divide both sides by 2
c = -8 is your result.
