10 pages is the starting point of Carla's book so it will be a constant, which means that 10 will be by itself with no variables next to it. The addition 15 per month will put next to the m since this is how much she is adding each month. So the equation will look like this:
P=15m+10
To find how many pages after 5 months, simply plug 5 into m and solve for P.