Problem 12.53 Acetic Acid vs. Formic Acid

hello,

Can anyone explain why formic acid is stronger than acetic acid? I don't fully understand what the solution means when it states that the CH3 group in acetic acid has electron-donating properties.

Please help.
Thanks!

I didn't find the right solution from the Internet.

References:
lavelle.chem.ucla.edu/forum/viewtopic.php?t=8659

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