Pardon the handwriting.

Consider both N amd F started from point O on the line. Since N travels faster than F, she can cover more distance over 2 hrs.

In 2 hrs, F ran a certain distance, while N can travel that same distance in a shorter time. The extra distance N travels is from her running faster, 3.5km/h faster.

This extra distance can be calculated by 3.5km/h x 2hr, which is 7km

Subtracting the total distance of 19km, with 7km, you get the distance travelled if they had both ran the same speed. Dividing this distance for the 2 of them, you get the distance travelled for F.

Find the speed of F and you can get the speed of N

Hope that helps! Go try it

Ask if still not clear :)