So what you want to find is: what number times 120% is $324.00. (IT is 120% percent because it is the original plus the new percent) Simply set it up like so:
SN *120% = $324.00
Now turn the percent into a decimal:
SN * 1.20 = $324.00
Now divide $324.00 by 1.20
SN = $270
The original value of the basketball player's autograph was 270 dollars!
Hope that helps!