For 3, you need to graph the equation of the line: y = -150x + 1800
What you can do is plot the x-intercept and y-intercept and then draw a straight line between them.
The y-intercept is (0,1800) because if you purchase zero shoes, you will have all $1800 left over.
The x-intercept is (12,0) because if you purchase 12 shoes, you will have zero dollars left over.
So plot (0,1800) and (12,0) for problem #3.
Problem #5, I already answered above. The x-intercept tells you how many shoes you can buy if you spend all your money (12 pair of shoes).