The boat went along the river for 5 hours, and then against the current for 3 hours. Find your own speed of the boat

The boat went along the river for 5 hours, and then against the current for 3 hours. Find your own speed of the boat if you know that the speed of the river is 3 km / h, but in total you have covered 126 km.

Let the boat’s own speed be x km / h. Then we write down the number of kilometers traveled along the river as ((x + 3) * 5), and the number of kilometers traveled against the river as ((x – 3) * 3). From the condition of the problem, it is known that the boat covered 126 km in total, we can draw up an equation and find the unknown:
((x + 3) * 5) + ((x – 3) * 3) = 126;
(5x + 15) + (3x – 9) = 126;
5x + 15 + 3x – 9 = 126;
8x + 15 – 9 = 126;
8x + 6 = 126;
8x = 126 – 6;
8x = 120;
x = 120/8;
x = 15.
Answer: the boat’s own speed is 15 km / h.