It is given that P is his total pay and N is the number of math is fun he sells.
It is also given that his base salary is $2100 and he gets $70 for every copy of math is fun he sells.
So the equation is:
So the equation is P=2100+70N.
If he sells 27 copies of math is fun then his pay will be:
He gets paid $3990 if he sells 27 copies of math is fun.