we are given the cost for 4 shirt s and for 12 shirts. Assuming that the shipping cost is constant, we can model the cost of the shirts as a linear equation. So, we want to find an equation of the form

where m is the slope of the line and b is the y-intercept.
Using the given information, we can wirte the following points (4,35) and (12,95). We will use this points to find the value of b and m.
Recall that the slopé of a line that passes through points (a,b) and (c,d) is described by the formula

in our case, we have a=4, b=35, c=12 and d=95. So we get

So, so far we have the equation

To find the value of b, we will use tghe fact that, as the line should pass through the point (4,35), this means that if we replace x by 4, we should replace y by 35. So we have that

so if we subtract 30 on both sides, we get

so the line equation would be

Now, we want to find the price for 10 tshirts. So we simply replace x=10 to find

so the price of 10 tshirts is 80