Question 1088766

{{{f(x)=x^2+28}}}

{{{f(3y)=2f(y)}}}

{{{(3y)^2+28=2(y^2+28)}}}
{{{9y^2+28=2y^2+56}}}
{{{9y^2-2y^2=56-28}}}
{{{7y^2=28}}}
{{{y^2=4}}}
{{{y=2}}} or {{{y=-2}}}

so, the one possible value of {{{y}}} is: C){{{2}}}