Explanation:
since I can't see any diagram, I have to assume that the tin can is a regular cylinder. that means top and bottom circular areas have the same size and are parallel to each other.
the circumference of a circle is
2×pi×r
in our case that is 8×pi cm.
so, that means the radius is 8pi/2pi = 4 cm.
the amount of metal needed is simply the sum of all surface areas (top and bottom and the side wall / mantle).
the areas of the top and bottom are circles. the area of a circle is
pi×r²
so, in our case, one circle has
pi×4² = 16pi cm²
top and bottom together have then
2×16pi = 32pi
now for the mantle :
when you cut off the top and the bottom, cut the remaining mantle straight down in one line, and then place it flat on a table, you have a rectangle.
the length is the original circumference of the top/bottom circles, and the width is the original height of the cylinder.
the area of the mantle is therefore
8pi × 8.5 = 68pi cm²
that means in total the surface (and the amount of needed metal) of the tin can is
32pi + 68pi = 100pi cm² = 314.1592654... cm²