${\mathrm{K}}_{\mathrm{a}}$ for  formic acid and  acetic acid  are $2.1 \times {10}^{-4}$ and $1.1 \times {10}^{-5}$ respectively. The relative strength of acids is

Relative strengths of weak acids = $\sqrt{\left(\frac{{\mathrm{K}}_{\mathrm{a}1}}{{{\mathrm{K}}_{\mathrm{a}}}_2}\right)}$

Assume $C_1 and C_2$ are same (Although not given).

$\therefore Relative strength = \sqrt{\left(\frac{{\text{K}}_{\mathrm{a}1}}{{\mathrm{K}}_{a2}}\right)} = \sqrt{\left(\frac{2.1 \times {10}^{-4}}{1.1 \times {10}^{-5}}\right)}$

Relative strength for HCOOH to $C{H}_{3}COOH = 4.37 : 1.$