1. The total surface area A exists of:

2 congruent right triangles which form a rectangle.(yellow)

2 rectangles (lightblue, green).

2. Unfortunately you didn't give us the dimension you have obtained by measuring

. So I called one side a and

**estimated **the missing length x as $\displaystyle x = \frac83 \cdot a$.

Then the hypotenuse of the right triangle is $\displaystyle x - a$

3. Use Pythagorian theorem to calculate the length of y:

$\displaystyle a^2+y^2=\left(\frac53 a\right)^2~\implies~y=\frac43 a$

4. Calculate the different areas:

$\displaystyle A_{blue} = a \cdot \frac83 a = \frac83 a^2$

$\displaystyle A_{green} = a \cdot \frac43 a = \frac43 a^2$

$\displaystyle A_{yellow} = 2\cdot \frac12 \cdot a \cdot \frac43 a = \frac43 a^2$

The sum of these 3 areas yields the total surface area $\displaystyle A=\frac{16}3 \cdot a^2$

5. If and only if you have measured a = 3 cm then the total area is indeed 48 cm².