We can use the formula for finding the length or magnitude of line joining two points.
Length = √((x2-x1)² + (y2-y1)²)
The points joining the line are C (-1,4) and D (2,0)
Length = √((2+1)² + (0-4)²)
= √((3²) + (-4)²)
= √ (9+16)
= √25
= 5 units
I am not 100% credible for this answer if it is incorrect, I do apologize.