Hit "work out the missing sides" to see how they're deduced.
The trap: an L-shape has two side lengths that aren't written on it, and one tempting line straight through the middle that isn't part of the outline at all. Work out the missing sides from the ones you're given, then add only the real edges going round.
Opposite sides of the whole L must match up: the two short top bits add to the long bottom, and the two short side bits add to the tall left. That's how you deduce the missing lengths — then trace the outline and add every edge (and never the dashed cut).