I do not see how the equation circled in red follows. Should it be (4l)^2 ≥ 225?
What is the area of the rectangle? It’s not 4l \times 4l.
Oh, I see. Do I have this right?
wl=225
w < 4l
Multiply both sides by l: wl < 4l
By substitution, 225 < 4l
So it follows 225 ≤ 4l
Indeed, though you meant wl < 4l^2 in the fourth line.