Ok, I don't know how you're suppose to solve this problem but one way to do it is to graph all the plots. X will be the square feet and Y will be the cost, I hope you know how to graph that. After you've done that you can either find a line of best fit by hand or using a TI calculator. If you have a TI calculator what you do is click stat, edit and insert x into L1 and Y into L2. After you entered that click stat plot, turn plot on for L1, L2 and then click stat go over to calc and click LinReg(ax+b) the equation you get is y=84.797x+14060.899 put 6000 in for x and you get 522846.053 if you aren't allowed to use calculator I would just half retricethe graph and use that answer and when you approximate the slope just try to make it look like you got something like 84.797. Anyways I hope that helps.