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