We must solve

Means the candles will be the same size in 4 hours
Undo what has been done to the variable. That means to do the opposite done to the variable.
5x-4=-19
+4=+4
5x=-15
5 is being multiplied to the variable so you divide it from both sides.
x=-3
You just need to know how much 10% is and then you just divided what 10% is and that will be equal to 5% and then you just add the 10 % and 5%