Answer
$A= 28$ square units
Work Step by Step
$\left|\begin{array}{lll}
a_{1} & b_{1} & c_{1}\\
a_{2} & b_{2} & c_{2}\\
a_{3} & b_{3} & c_{3}
\end{array}\right|$ = see p.645...
$=a_{1}b_{2}c_{3}+b_{1}c_{2}a_{3}+c_{1}a_{2}b_{3}-a_{3}b_{2}c_{1}-b_{3}c_{2}a_{1}-c_{3}a_{2}b_{1}$
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Using the given formula with
$(x_{1},y_{1})=(3,-5)$
$(x_{2},y_{2})=(2,6)$
$(x_{3},y_{3})=(-3,5)$
$\left|\begin{array}{lll}
3 & -5 & 1\\
2 & 6 & 1\\
-3 & 5 & 1
\end{array}\right|=18+15+10-(-18)-15-(-10)$
$=71-15=56$
$A=\displaystyle \pm\frac{1}{2}(56)$
$A= 28$ square units
