y=0.4x+b, using either point we can now solve for b, the "y-intercept", which in this case is the weight at birth :) I'll use point (1, 7.2) so our equation is:
To find your answer you would divide 21 by 3 which would be 7, once you've got 7 you would multiply it by 9 which would give you the amount of old houses that there is which would be 63.
