Formic acid has a pKa of 3.75 and acetic acid has a pKa of 4.76. Why do we consider formic acid to be a

"stronger" acid than acetic acid? Please include explanation in mathematical terms.

It's like a rule....
The lower the pKa the higher the association, thus being more acetic.
Recall that the higher the Ka (not pKa) the higher the dissociation.
-log(Ka) = pKa

So,
Ka = 10^-(pKa)

Test it out yourself....
10^-3.75 = .0001778
10^-4.76 = .000017378

See?

Any confusions or questions just let me know. I'd be glad to help you out.
Goodluck.