An unknown quadrilateral has these points: A (0, -3), B (6, -3), C (9,1), D (3,1)

The length of side BC can be found using the distance formula, which is:

Distance = √((x2 - x1)^2 + (y2 - y1)^2)

For points B (6, -3) and C (9, 1), the distance formula becomes:

BC = √((9 - 6)^2 + (1 - (-3))^2)
= √(3^2 + 4^2)
= √(9 + 16)
= √25
= 5

Therefore, the length of side BC is 5.