Crikey. If I had known trigonometry and calculus would have been this important, so many years after college.
And I just realized - reading the problem again - I goofed.
Line segment AB² = (½ Diversion Street Width)² + (Distance from Continuous Street up Diversion Street)² -or-
Line Segment AB² = (10²) + (25²) -or-
Line Segment AB² = (100) + (625) -or-
Line Segment AB² = 725 -or-
Line Segment AB = 26.925824
Sorry, Oscar!