Question 882262
Not sure what level since you did not say.


Cutting the open cylinder along its length and unroll, you find the unrolled length corresponds to the circumference of the rolled form; and the length along which the cylinder was cut is "height" of this unrolled form.  You also have the two disc surfaces.


You have then two terms to be added for the total surface area:
{{{h*(2pi*r)+2(pi*r^2)}}}, the sum of the unrollable curved tube and the two circular discs.  You know h=21, and r=4, so you can replace the variables with them.


We have Commutative Property of Addition, so you can also say {{{2(pi*r^2)+h(2pi*r)}}}.