5x+12z Because you take, 3z+9z and that is equivalent to 12z. and then you put the 5x back into the equation so it is 5x+12z
It sounds like initially we have a total of 18*5 = 90. Then we are adding 5 + 4 +3 +2 +1 to this.
so that gives us 105.
105/5 = 21
