Mass of an aspirin tablet is 0.42g and contains 77.38% ASA by mass.

I dissolved the tablet in hot naOH and cooled the solution with distilled water to the mark of 250 volumetric flask.
Then I pipetted 5.0 mL of the solution to the 100 mL volumetric flask and I diluted it with FeCl3 solution to the mark.

How do I find the concentration of the solution?

concn in what? In the final 100 mL solution?
0.42g x 0.7738 = g ASA in 250 mL.
You took 5/250 and diluted to 100. That is g/100 mL.
If you want molarity, then mols/L = M.
Post your work if you get stuck.

How can I get the cocnentration mg/L?

You have g/100. Let's just say that is 0.12 g/100 mL. Then, that is
0.0012 g/mL. Convert to g/L.
0.0012 g/mL x 1000 mL/L = 1.2 g/L
Convert to mg.
1.2 g x 1000 mg/g = 1200 mg/L.

Oh alright! Thank you!

You're welcome! I'm glad I could help. If you have any more questions, feel free to ask!