42000 - R1 + 6R1 = 0.0042 * 7000 R1 - 0.0041 R1^2 ?
what is R1
You'll get a better solution if you start off with the right algebra. You should have
6/R1 + 6/(7000-R1) = .0042
6(7000-R1)+6R1 = .0042*R1(7000-R1)
42000 - 6R1 + 6R1 = 0.0042*7000*R1 - 0.0042*R1^2
.0042 R1^2 - 29.4 R1 + 42000 = 0
R1^2 - 7000 R1 + 10,000,000
That should give you better results. It might be easier to do the math using scientific notation...