Look at the pic yea it's hard sorry but i didn't listen to the teacher

This is a triangle 
To find the area, count the side (which is 5)
You can see that the peak opposite of the side is 7 units in length, that is the height
The formula for the area of the triangle is 
A = (b * h)/2
b = 5
h = 7
A = (5 * 7)/2
A = 35/2
A = 17.5

Hope this helps

17.5 take the base(5) and multiply it by half the height (7 divided by 2= 3.5).