First you find the area of the front face 1/2bh or in you case 10x12=120 1/2 of 120 is 60 next you multiply 60 by 20 to get 1,200 for the voulume
Step-by-step explanation:
lets take a look....
In order to find the perimeter of a triangle you have to get the sum of all sides. In other words,
Blue triangle:

Red triangle:

Difference:

sorry I don't know the answer but i hope you get well soon