to get the concentration of your sample solution, you should take your sample absorption ( = y) and then calculate the cation concentration of your sample solution (= x). The correct formula is x = (y-b)/a. Then you should calculate the ammount of cations which is dissolved in 50 ml by multiplication with the correct factor . This result is the ammount of your cation (for which you have done the calibration) in 1 g of your sample.
After the AAS measurement you will get the concentration of the prepared sample in ppm (mg/L). multiply the result by the sample solution in L (50/1000 in your case) then multiply by 1000 to covert to microgram then divide on the solid sample weight (1g) to get the concentration of the cation in µg/g which is mg/Kg or ppm. This is the correct factor mentioned by Timm Wolf (50 in your case)
From a linear curve, you will get the concentration of cation in the sample solution in mg/L (ppm). You can find out the concentration of cation in the plant tissue in mg/kg (ppm) using the following formula:
M = C. V / W
Where, M = Concentration of cation in the plant tissue in mg/kg (ppm)
C = Concentration of cation in the sample solution in mg/L (ppm)
V = Made up volume of sample extract in mL
W = Weight of plant tissue taken for extraction in g
If you dilute the sample solution before AAS measurement, multiply the result with dilution factor (e.g., if you dilute 1ml sample solution to 10ml, then multiply the result with the factor 10.