Use the information in the problem to set up
y = mx + b equations for both Frank and John.

Set the equations equal to each other and solve for x.

Substitute x back into one of the original equations and solve for y. Check that x and y work in both equations.

Define your variables.

y = total weight (lbs)
x = time (weeks)
m = weight growth each week (lbs/week)
b = starting weight (lbs)