Question

why formic acid is stronger than acetic acid

Answer 1

because the methyl group is electron donating in the conjugate base of acetic acid, this destabilizes the conjugate base by exacerbating the existing negative formal charge on the deprotonated oxygen, while in formic acid the electron donating methyl is absent in lieu of a hydrogen

Answer 2

In acetic acid, a methyl group is attached to the -COOH group , this methyl group show electron donating nature because of which the acidic strength of the -COOH group gets deceased.
In Formic acid, this methyl group is not present and a H atom is attached to -COOH group which do not show any such electron releasing nature. 
So Formic acid is more acidic than acetic acid.

may this helps u :-)