The boat covered 20 km along the river and the same amount back, spending 1 hour 45 minutes

The boat covered 20 km along the river and the same amount back, spending 1 hour 45 minutes for the whole journey. The river speed is 2 km / h. Find your own boat speed.

Let’s say that the boat’s own speed is a km / h.
This means that along the course of the river it was equal: a + 2 km / h.
Upstream of the river a – 2 km / h.
1 hour 45 minutes = 1.75 hours.
We get the equation:
20 / (a + 2) + 20 / (a – 2) = 1.75.
20 * a – 40 + 20 * a + 40 = 1.75 * a ^ 2 – 7.
1.75 * a ^ 2 – 40 * a – 7 = 0.
D ^ 2 = (-40) ^ 2 – 4 * 1.75 * (-7) = 1600 + 49 = 1649.
D = 40.6.
x = (40 + 40.6) / 2 * 1.75 = 80.6 / 3.5 = 23.0285 km / h.
We take our own speed equal to 23 km / h.
Answer: The own speed is approximately 23 km / h.