Adding A Strong Acid to a Buffer Chemistry help needed!!!!!!!?

A beaker with 120 mL of an acetic acid buffer with a pH of 5.00 is sitting on a benchtop. The total molarity of acid and conjugate base in this buffer is 0.100 M. A student adds 4.00 mL of a 0.420 M HCl solution to the beaker. How much will the pH change? The pKa of acetic acid is 4.760.Adding A Strong Acid to a Buffer Chemistry help needed!!!!!!!?
let x = [acetate]

ley y = [acetic acid]



x + y = 0.100

5.00 = 4.760 + log x/y



5.00 - 4.760=0.24 = log x/y

10^0.24 =1.74 = x/y

1.74 y = x



1.74 x + x = 0.100

x = 0.0365

y = 0.0635



moles acetate = 0.0635 x 0.120 L=0.00762

moles acetic acid = 0.0365 x 0.120 L=0.00438



moles H+ added = 4.00 x 10^-3 L x 0.420 M=0.00168



CH3COO- + H+ = CH3COOH

moles acetate = 0.00762 - 0.00168=0.00594

moles acetic acid = 0.00438 + 0.00168=0.00606



pH = 4.760 + log 0.00594/ 0.00606=4.75



delta pH = 0.25