Because, it doesn't follow the rule c=5. All the problems need to abide by c=5. So the error is 5c-6=4-3c. If you do need to change it, you just need to change the 3c in the problem into -3c.
Basically the error is that the 3c in the problem is positive not negative.
The other problems are correct and equal. This one is wrong because when you do the equations you get 19 = -11 which is clearly not equal. You would need to change the equation to make it equal. So it would be:
5(c) - 6 = 4 - -3(c) so now it is 19 = 19.
So you would change 5(c) - 6 = 4 - 3(c) into 5(c) - 6 = 4 - -3(c)
