Answer: 90 cm


Label the blocks    and say the height of the table is $h$

  $84+l=h+w$                                        $96+w=h+l$

$l$ and $w$ are on different sides of the equations so add to make them cancel: $$\begin{align}&\qquad 84+l=h+w\ \ \\
&\underline{+\quad 96+w=h+l\ \ }\\
&180 + l+w=2h+l+w\end{align}$$ so $2h=180$ and $h=90$