a box has a volume of 750cm3 and a surface area of 2600cm3. What are the length, width, and height of the box

First the surface are must be in square units.    

l*b*h=750   (1)

2(lb+lh+bh)=2600

lb+lh+bh=1300

I do not think that you can solve this because 3 unknowns usually needs 3 equations to solve.

$$\underset{\,\,\,\,{\textcolor[rgb]{0.66,0.66,0.66}{\rightarrow {\mathtt{lbh, lb, lh, bh}}}}}{{solve}}{\left(\begin{array}{l}{\mathtt{Lbh}}={\mathtt{750}}\\
{\mathtt{Lb}}{\mathtt{\,\small\textbf+\,}}{\mathtt{Lh}}{\mathtt{\,\small\textbf+\,}}{\mathtt{bh}}={\mathtt{1\,300}}\end{array}\right)} \Rightarrow \left\{ \begin{array}{l}{\mathtt{lbh}} = {\mathtt{750}}\\
{\mathtt{lb}} = {\mathtt{\,-\,}}{\mathtt{r20}}{\mathtt{\,-\,}}{\mathtt{r19}}{\mathtt{\,\small\textbf+\,}}{\mathtt{1\,300}}\\
{\mathtt{lh}} = {\mathtt{r20}}\\
{\mathtt{bh}} = {\mathtt{r19}}\\
\end{array} \right\}$$

I have no idea what thid calculator has done   

Melody is correct......there will be infinite solutions for this......this will happen with more variables than equations.....