1.9 x 10-3=((6.67 x 10-11)(8.1 x 102)(4.5 x 108))/r2

1.9 x 10^-3=((6.67 x 10^-11)(8.1 x 10^2)(4.5 x 10^8))/r^2, solve for r
 0.0019 = 24.3122/r^2 cross multiply
0.0019r^2 =24.3122     divide both sides by 0.0019
r^2 =24.3122 / 0.0019  
r^2 = 12,795.89              take the sqrt of both sides
r = 113.12