linear growth question: I'm not too comfortable with my set up for this equation, so anyone pls show how they would set up this..??

Let ( t , w ) denote t weeks and w lbs.
m = slope of the line

By these three points ( 0 , 7 ) , ( 4 , w ) , ( 20 , 12 ) , we have
m = ( w - 7 )/( 4 - 0 ) = ( 12 - 7 )/( 20 - 0 )
( w - 7 )/4 = 5/20 = 1/4
w - 7 = 1
w = 8

Ans : 8 lbs