12.6
1) Since we have a radius of 4 (half of the diameter), and a height of 12, then we can find the slant height by using this formula:
s² =h² +r² Note that it is derived from the Pythagorean Theorem.
√s²=√h²+√r²
s=√12 +√16
s=√12 +4
s ≈ 12.65 units
2) Rounding off to the nearest tenth we can say the slant height is 12.6