You use the Theorem of Pythagoras:
Imagine the Car is driving: when it turns left to drive north This has to be an 90 degree angle:
^
---> |
But the displacement is Always the shortest Line between the start and the end and in This case it is a Line which creates a triangle.
In a triangle With a right angle you can use Pythagoras:
215^2+45^2=c^2
v=√(215^2+45^2)