Kristen can run 9km/h and walk 3km/h. she completed a 42km marathon in 6 hours. what distance did kristen run during the marathon?

Let
 x = distance traveled running.
 y = distance traveled walking.
 x + y = 42
 1 / 9x + 1 / 3y = 6
 Solving the system of equations:
 -3 * (1 / 9x + 1 / 3y) = - 3 * (6)
 -1 / 3x + -y = -18
 Then,
 x + y = 42
 -1 / 3x + -y = -18
 Adding up:
 -1 / 3x + x = 24
 2 / 3x = 24
 x = (3/2) * 24
 x = 36 Km
 
 kristen ran 36 Km during the marathon

It would be 15 m how i got the answer: (20x20) + b^2 = (25x25)400 + b^2 = 625625-400=225square that and get 15hope this : )

The answer to your question is b