let

v = 10 km/h the speed of the ship

t1 = 3 hr the time of the travel north

t2 = 7 - 3 = 4 hr the time of the travel east (since the total time is 7 hr)

d = the distance that we need to find

if we make a drawing of the path of the ship we see that he travels on the legs of a right triangle and the distance that we need to find is the length of the hypotenuse of this triangle.

since speed = distance/time follows distance = speed*time and the legs of the right triangle are:

v*t1 = 10*3 = 30 km

v*t2 = 10*4 = 40 km

d^2 = 30^2 + 40^2

d^2 = 2500

d = 50 km

the ship is 50 km far from the starting point.