Question 162512
Let's graph the points.
{{{drawing( 300, 300, -12, 2, -12, 2,grid( 1 ),circle( -8, -2, .2 ),
circle( -1, -11, .2 ),
circle( -1, -2, .2 ),
green(line(-1,-11,-1,-2)),
green(line(-1,-2,-8,-2)),
green(line( -8, -2, -1, -11)))}}}
.
.
.
The area of a triangle is 
{{{A=(1/2)(bh)}}}
where b is the base, h is the height of the triangle.
From the diagram,
{{{b=7}}}
{{{h=9}}}
{{{A=(1/2)(bh)}}}
{{{A=(1/2)(7*9)}}}
{{{A=63/2=31.5}}}
Your answer is correct.