The point (12,-8) has the x coordinate of x = 12
Going from x = 3 to x = 12 is a distance of 9 since 12-3 = 9
Therefore this square has a side length of 9 units
There are 4 of these congruent (same length) sides, so 4*9 = 36 is the perimeter
Answer: B) 36
Note: you can use the y coordinate if you want. The distance from y = 1 to y = -8 is also 9 units because 1-(-8) = 1+8 = 9