Calculating [HI] given Kc, [H2], and [I2] for the reaction 2HI  H2+I2
You're close. Using the expression for $K_c$, you just need to solve for $[\ce{HI}]$:

$$K_c = {[\ce{H2}][\ce{I2}]\over[\ce{HI}]^{2}}$$

$$0.0198 = {(\pu{0.012 M})(\pu{0.015 M})\over[\ce{HI}]^{2}}$$

$$[\ce{HI}] = \sqrt{{(\pu{0.012 M})(\pu{0.015 M})\over 0.0198}} = \pu{0.0953 M}$$