Let x be her speed for the first part of the journey.
We know:
Time = distance / speed
For the first part:
time = 350/x
For the second part:
time = (470 - 350)/(x - 15) = 120/(x - 15)
We are told the times are equal so:
350/x = 120/(x - 15)
350(x - 15) = 120x
230x = 5250
x = 22.8 km/hr
Speed for second part = 22.8 - 15 = 7.8 km/hr