First since it is a triangle and it says meter square that means its an area
so the area is 1/2hb h stands for height and b stands for base
so
a=1/2bh
a=1/2(12)(9)
a=1/2(108)
a=54m^2
It says the cost of sail is $15 per square meter
so
$15*54
=$810
Why times by 54 because if 1 piece is $15 how about 54pcs?
thats why we will multiply it