Help! I'm not sure how to do it

You can split it to a triangle along with a rectangle underneath.
Formula for calculating triangle:
height x base divide by 2
Firstly we will have to calculate the triangle.
0.8x1.8/2 ( since 1.8 matches the base of the triangle we are looking for) which equals to =0.72 m
Now let’s calculate the rectangle:
1.8 x 2.1 =3.78

Finally add them both together which is:
0.72+ 3.78= 4.5 m