I am not sure about the linear equation format but I used
1,775,721 = 5.879x + 1.704x + x
to get an estimated population of
206,886.151
Input 15 as the initial value and (3, 90) as x and y to get the equation 90=m(3)+15. Isolate m to get m=25. Each shirt costs $25 and the equation that represents the scenario is y=25x+15.