Answer: The magnitude is 8.66 blocks
Step-by-step explanation:
First, the cab moves 5 blocks to the north, after that, the cab moves 5 blocks to the east, and after that, the cab moves another 2 blocks to the north. 
So the total displacement is 7 blocks to the north and 5 blocks to the east, 
But the total displacement will be the hypotenuse of a triangle where the cathetus is those two values: 
As you know, for the Pythagorean's theorem we have that: 
H^2 = 7^2 + 5^2 = 49 + 25 = 74 
H = (75)^(1/2) = 8.66 blocks 
Then the magnitude of the taxi's displacement (the difference between the final position and the initial position) is 8.66 blocks