The smaller triangle is the image of the larger triangle after a dilation. The center of the dilation is (4,2).

We have that

< -8, 2>  =   [  <8, 2> - <4,2> ] * F   +  < 4,2>         where F is the scale factor

  ( < -8 , 2 >  - < 4, 2 >  )  =   <4,0 > * F

< - 12,  0  >   = < 4 , 0 > * F

So  F  = - 3    = the scale factor