Verified answer

It is a buffer solution.

we can use the equation

pH = pKa + log [ sodium benzoate ] / [ benzoic acid ]

pKa = - log Ka = 4.17

pH = 4.14 + log 0.50 / 0.75 = 3.96

use the concept of buffer solution

here it's an acidic buffer

An acidic buffer solution is simply one which has a pH less than 7. Acidic buffer solutions are commonly made from a weak acid and one of its salts - often a sodium salt.

pH=pKa+log[salt ]/[acid]

pH=3.99

for more help follow this link

http://en.wikipedia.org/wiki/Henderson-Hasselbalch...

and also read the buffer concept