The road between points A and B consists of an ascent and a descent, and its length is 34.6 km. The cyclist covered the path

The road between points A and B consists of an ascent and a descent, and its length is 34.6 km. The cyclist covered the path from A to B in 4 hours, of which the descent took 1 hour. With what speed the cyclist rode downhill, if his speed uphill is less than the speed downhill by 1.8 km / h.

X is the length of the descent, then
1.8+ (34.6-x) / 3 = x / 1
We bring everything to a common denominator and get:
40-4x = 0
X = 10
Verification:
1.8+ (34.6-10) / 3 = 10
1.8 + 8.2 = 10
10 = 10