Answer
$H_2CO_3$ is a weak acid.
$$K_a = \frac{[HC{O_3}^-][H_3O^+]}{[H_2CO_3]}$$
Work Step by Step
$H_2CO_3$ does not appear in the common strong acids list:
$$Strong \space acids: HCl, HNO_3, H_2SO_4, HBr, HI, HClO_4 \space and \space HClO_3$$
$$H_2CO_3(aq) + H_2O(l) \leftrightharpoons HC{O_3}^-(aq) + H_3O^+(aq)$$
$$K_a = \frac{[Products]}{[Reactants]} = \frac{[HC{O_3}^-][H_3O^+]}{[H_2CO_3]}$$
