Weak acids and bases: formic acid vs. acetic acid

[Beverly Lauring 1L]

One of the textbook questions asks to suggest an explanation for the different strengths of acetic acid and formic acid. The correct answer is that formic acid is a stronger acid since the -CH3 group in acetic acid has electron-donating properties. I do not understand how we know it has electron-donating properties. How do you compare the carboxyl group in formic acid to the Ch3 group in acetic acid?

[Nicholas Ryan 2K]

Methyl groups (CH3-) donate electrons because they are centered around carbon, which isn't very electronegative, and so it does not "hold on" to it's electrons very tightly. Molecules that donate electrons are Lewis bases, and this basic character of the group reduces the acidity of acetic acid.