k = 4
1) Examining each leg of those triangles we can say that
The black one (larger): has its base measuring 12 units
The orange one (smaller) has a base whose width is 3 units
2) If we count the vertical and horizontal displacement of the greater one, we can state that the black shape is the dilated version, with a scale factor of 4
3) Then the scale factor is 4
Because the other legs of the black shape are 4 times greater than the orange one.