It means: It is known that A and B start from the same place at the same time, with the speed of A being 7 and the speed of B being 3. B go straight east. A first walks 10 steps to the south, and then walks obliquely to the north and east for a while before meeting B. So how far did Party A and Party B walk when they met?

3t * 3t+ 10 * 10 =(7t- 10)*(7t- 10)

T = 3.5, then 3t = 10.57t = 24.5.

T is time, because A and B walk at the same time, regardless of the angle, but there must be right angles, and the squares of the two right angles are equal to the square of the hypotenuse, so 10 2+(3t) 2 is equal to the hypotenuse (7t- 10) 2.