Question

The standard solution of FeSCN2+ (prepared by combining 9.00 mL of 0.200 M Fe(NO3)3 w/1.00 mL of 0.0020 M KSCN) has an absorbance of 0.510. If a trial's absorbance is measured to be 0.250 and its initial concentration of SCN– was 0.00050 M, the equilibrium concentration of SCN– will be ?

I'm not sure I understand what you did; however, if I understand correctly, then
(std) = 0.002 M x 1/11 = ?? M
Then A=abc. Let's call b = 1 and evaluate a.
Use A = 0.510; b = 1, c from above, and evaluate the constant a.

Then redo, using the constant of a you just found, A of 0.250, b = 1, and evaluate c, concentration.
Check my thinking.