$($c$)$ Flame atomic absorption spectroscopy is used to determine copper in a soil sample. $3\mathrm{.}5g$ of a soil sample was digested, filtered and the solution made up to $250c{m}^3.$ Next $10c{m}^3$ of this solution was made up to $50c{m}^3,$ containing a standard copper solution $(12ppm$ as outlined in the table below

$\backslash$table$[[$Volume of unknown $(c{m}^3),$Volume of $12ppm$ standard $(c{m}^3),$absorbance$],[10,0,0\mathrm{.}201],[10,5,0\mathrm{.}291],[10,10,0\mathrm{.}378],[10,15,0\mathrm{.}467],[10,20,0\mathrm{.}554]]$

Use a graphical method to determine the copper levels in the soil in ppm$.$