the surface area is the sum of the circle at the top, the circle at the bottom and the rectangle that "covers the cylinder", like the label of a can if that makes sense.
the surface area for each circle is
S= π × r²
where r is the radius, this is diameter/2
the surface area for the rectangle is
S= b × h
where the base is the diameter and the height comes from this formula
hyp² = op² + adj²
because you have a right triangle where your hypotenuse is 20cm, your opposite cathetus is the one you need because is theheight of the cylinder , and the adjacent cathetus is the diameter
hope this helps, the answer should be 507.68cm²