For this problem, we have to write equations that represent the heights of the 3 objects in the picture (cat, table and turtle). So, from the image on the left, we have: cat + table - turtle = 170.
From the image on the right, we have: turtle + table - cat = 130
Adding the 2 sides of each equation, we have: (cat + table - turtle) + (turtle + table - cat) = 300 => 2 tables = 300 => table = 150.