Hello Julian.Ignacio!

6x+3=2x-7

add +7 to both sides:

6x + 3 +7 = 2x - 7 + 7

6x + 10 = 2x + 0

add -6x on both sides:

6x +10 - 6x = 2x - 6x

10 = -4x

divide by -4

- (10/4) = x

-10/4 is equal to -5/2

I give you a hint: because one hiker goes north 3km/h, the other goes south 4km/h, it is like there is only one hiker traveling into one direction with (3+4)km/h = 7km/h.

Then you want to know how long it takes to reach the 10km mark.

So you need to solve

7 (km/hours) * x (hours) = 10 (km)

Yours

Rapha