Table height

Answer: 90 cm

Label the blocks  

Image

  and say the height of the table is $h$

Image

  $84+l=h+w$                                      

Image

  $96+w=h+l$

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