Answer: 13 units
The x value goes from x = 21 to x = 9, which is an x distance of 12 units (21-9 = 12)
The y distance is 5 units (16-11 = 5)
We have a right triangle with legs of 12 and 5. The hypotenuse is x
Use the pythagorean theorem to find x
a^2 + b^2 = c^2
5^2 + 12^2 = x^2
25 + 144 = x^2
169 = x^2
x^2 = 169
x = sqrt(169)
x = 13
The hypotenuse of this right triangle is 13 units, so this is the distance between the two points.