7.50 mL of a 0.150 M solution of NaOH was added to 20.00 mL of a weak acid, HA. The pH of the resulting solution was 3.50. KA of the acid is 2.82 x 10^-4 . Calculate the concentration of the acid used to prepare the solution. Use the simultaneous equation method for the calculation. Note any simplifying assumptions made in the derivation.