Respuesta :
d = 15.
The easiest way to calculate the distance between two point in the space is usign the equation d = √(x₂-x₁)²+(y₂-y₁)².
The distance between the points (3, 9) and (12, 21) is:
d = √(12-3)²+(21-9)² = √(9)²+(12)² = √81+144 = √225 = 15