The magnitude of vector $ \vec{v} $ is $ \| \vec{v} \| = 5 $ .
To find magnitude of a vector $ v=(a,b) $ we use formula $ ||v|| = \sqrt{a^2+b^2} $
In this example $ a = 3 $ and $ b = -4 $, so:
$$ ||v|| = \sqrt{ 3^2 + (-4)^2 } = \sqrt{ 9 + 16 } = \sqrt{ 25 } = 5$$