Set up an ICE table based on concentrations in mol/l:
$sf(" "CH_2H_3O_(2)+H_2OrightleftharpoonsC_2H_3O_2^(-)+H_3O^(+))$
$sf(color(red)(I)" "0.210" "0" "0)$
$sf(color(red)(C)" "-x" "+x" "+x)$
$sf(color(red)(E)" "(0.210-x)" "x" "x)$
Applying the equilibrium law:
$sf(K_c=(x^2)/((0.210-x))=1.8xx10^(-5))$
Because the dissociation is so small we can make the approximation:
$sf((0.210-x)rArr0.210)$
$:.$$sf(x^(2)=0.210xx1.8xx10^(-5)=0.378xx10^(-5))$
$:.$$sf(x=sqrt(0.378xx10^(-5))=1.9xx10^(-3)color(white)(x)"mol/l")$