Answer:
6 inches
Step-by-step explanation:
Formula for surface area of a cylinder:

Plug in values and solve for the radius:

Divide both sides by 

Subtract 90 from both sides

Use the quadratic equation to find the roots
±
(sorry for formatting, Brainly doesn't
allow equations with ± in them)
and 
and 
and 
and 
and 
and
We can't have a negative radius, so that leaves us with r=6 in.