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ok lets try:

Table + cat - turtle = 170
Table - cat + turtle = 130
—————-+
2 tables = 300
Table = 150

Let's see if I've got it right (didn't look for other answers).

Turtle = x
Cat = y
Table = z

y + z - x = 170
z - y + x = 130
_ _ _ _ ... _ _ _ _
2 z = 300
z = 150 cm

Table height=h
Cat height= x
Turtle height=y

h+x-y=170
h-x+y=130

By adding(x and y cancels out), 2h=170+130
=> 2h=300
=> h=150