DECA=10, decagon = 10 sides.
since each side is 1.8 long, all 10 sides will be 10*1.8 or 18 units long, so that'd be the perimeter then,
[tex]\bf \textit{area of a regular polygon}\\\\
A=\cfrac{1}{2}ap~~
\begin{cases}
a=apothem\\
p=perimeter\\
------\\
a=2.8\\
p=18
\end{cases}\implies A=\cfrac{1}{2}(2.8)(18)[/tex]
