Let's assume
number of T-shirts is x
Firstly , we will find cost function
Calculation of cost function:
A high school purchases a printing machine that can put lettering on clothing for $6,000
so, cost of printing machine =$6000
initial cost =6000
Each T-shirt costs the school $4
so, total cost =6000+4*x

Calculation of selling price function:
we are given
Each T-shirt costs the school will sell for $12
so,
total selling price =12x

Break-even point:
In break-even point , selling price = cost price
so,

now, we can plug value

now, we can solve for x



So, the number of T-shirts are 750.........Answer