Step-by-step explanation:
to work out surface area you need to figure out the area of each face on the shape.
the first side is 7.5 x 9 = 67.5
there are two of these sides
this makes 135mm
the top side is (4.5 x 6) / 2
this makes 13.5
there are also two of these sides making 27mm
finally the back side is 4.5 x 9 = 40.5
there are only one of these sides
this makes 40.5mm
finally you need to add all these sides
135 + 27 + 40.5 = 202.5
the surface area is 202.5mm
