You are right, it is $75.
If you draw a line through the points, you can see that the line is linear, meaning that the line has the same slope. The slope of the line is 3/2 or 1.5, because from each point, you are going up 1.5 units, and to the right 1 unit. From the 4th week, if you go up 1.5 units and to the right 1 unit, at the 5th week your y value will be 75.