Alright, here's what we know.
He travels 2 hours at a fast speed, lets call that speed f km/h
He then travels 3 hours at (f-10) km/h
In these 5 hours, he travels 90km
So we have the formula velocity = distance/time. If you don't know where that formula comes from, let me know and I'll explain.
So we have:
d and t are given to us as 90 and 5 respectively
v is going to be his average speed, so we take 2 times the fast speed plus 3 times the slower speed and divide it by 5. This gives us the average speed he travelled over the 5 hours.
by cancelling the 5's in the denominator and expanding
So his faster speed is 24 km/h. Let me know if this solution did not make sense to you at any point.