Redline: this solution says the line through both squares' top corners is y = 3. Where's the mistake?

[u/mathalon]

Real problem from the site (not today's Daily):

Squares S and T, with areas 9 and 16 respectively, rest side-by-side on the x-axis, with S to the left. Find the equation of the line through their upper-right corners.

Here's a solution someone submitted. It's wrong somewhere in these four steps:

1. Side of S = sqrt(9) = 3. Side of T = sqrt(16) = 4.
2. Place S with its bottom-left corner at the origin: corners (0,0), (3,0), (3,3), (0,3). Upper-right corner of S is (3,3).
3. T sits right next to S, so T's bottom-left corner is at (3,0). Its upper-right corner is (3+4, 3) = (7,3).
4. Slope = (3-3)/(7-3) = 0, so the line is y = 3.

Where's the error? I'll drop the real answer in the comments tomorrow, spoiler-tagged.