Question 751782
Say the diameter is d and the radius is {{{d/2}}}.

Let h= height of the cylinder.


{{{h=3*d}}}


circumference is the LENGTH of the rounded side.  This circumference is {{{2*pi*(d/2)}}}, or 
circumference is {{{d*pi}}}


The surface area then is {{{h*(d*pi)=3*d*(d*pi)=3d^2*pi}}}
and you were given in the description:
{{{highlight(3d^2*pi=126)}}}.
Solving for d is uncomplicated.