Question

how can you prove newtons third law of motion by using spring balences given to you.

Answer 1

You can take 2 spring balances, attach one end of a balance to a wall. Join both the balances in front using the latch.
Now as you pull the free balance, u get a same reading in both of them, but direction being opposite; hence, verifying the law.

Answer 2

Experiment Let the ring of a spring balance B attached to a hook fixed in a wall, and then the hook of another spring balance A is attached to the hook of the spring balance B. Now the ring of spring balance is pulled. We find that both the balances show the same reading.

Explanation The spring of balance A pulls the spring of balance B due to which we get some reading in balance B. The same reading is seen in balance A because the spring of balance B also pulls the spring of balance A by the same force. The pull on the spring B by the spring A is the action and the pull on spring A by the spring B is the reaction.