Calculate the pH of a buffer made with 0.45M acetic acid and 0.75M sodium acetate. Ka = 1.8 x 10-5.?

Use the expression for the ionisation constant of ethanoic acid, Ka = [H+][CH3COO-]/[CH3COOH] so [H+] = 1.8 x 10^-5 x 0.45/0.75