I assure you that this is not a homework problem, I’m a nearly 40-year-old computer programmer just trying to measure plywood to cover a window to protect it from hurricanes.
Please see this line-art drawing of the window:
I have all the outer measurements (in the larger font), and 25" is the measurement from the bottom leftmost corner to 25 inches in, to the first red line.
I do not remember how to figure out how long the first red line is.
I’d simply make the whole thing and then split it into thirds afterwards, but the plywood sheets aren’t big enough to do this, I have to make the “thirds” from scratch.
I tried to find the solution to this on Geometry web sites, but no luck.