Write out the Eq. for slope:
[tex]m= \frac{( y_{2} - y_{1} )}{( x_{2} - x_{1})} [/tex]
(-4,8) = (x1,y1)
(r, 12) = (x2,y2)
m=[tex] \frac{4}{3} [/tex]
Use this information to plug into the slope equation:
[tex]m= \frac{( y_{2} - y_{1} )}{( x_{2} - x_{1})} [/tex]
[tex] \frac{4}{3} = \frac{( 12 - 8 )}{( r - (-4))} [/tex]
[tex]\frac{4}{3} = \frac{( 4 )}{( r - (-4))}[/tex]
Since we want the bottom of the fraction =3 set r-(-4)=3 and solve
r-(-4)=3
r+4-4=3-4
r= -1
