the area of a rectangle is just the product of its length and its width.
let's firstly convert the mixed fractions to improper fractions, and then multiply.
[tex]\bf \stackrel{mixed}{10\frac{9}{12}}\implies \cfrac{10\cdot 12+9}{12}\implies \stackrel{improper}{\cfrac{129}{12}}~\hfill \stackrel{mixed}{8\frac{1}{2}}\implies \cfrac{8\cdot 2+1}{2}\implies \stackrel{improper}{\cfrac{17}{2}} \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ \cfrac{129}{12}\cdot \cfrac{17}{2}\implies \cfrac{43}{4}\cdot \cfrac{17}{2}\implies \cfrac{731}{8}\implies 91\frac{3}{8}[/tex]
