Well this question uses the Pythagoras' Theorem (Which I have done a lot on!)
So lets start with the equation the make it easier.
A²+B²=C²
Now we implement where A, B and C are on the triangle. With going north it would be A, going east is B, so we are trying to find the hypotenuse or C.
We then incorporate the numbers on the triangle to the equation so it would look like this:
4² (A²) + 3² (B²) = C²
Then we make sure the only indices is with the letter C:
16 + 9 = C²
We then add 16 and 9 together to find what C² is.
25 = C²
Now we do the inverse to squaring, which is square root.
square root of 25 is 5.
5 = C
There ya go.