Question 1087404
You first want to convert the words to an algebraic expression for ease. We can break this down bit by bit.

"Length of hall is"
We can conclude here that the expression is an equality; something equalling something else. For simplicity, we can call the length of the hall L. We then have the following:
L=

"12 m less"
In English, something being 12 m less than something else is smaller by 12m. 12m less means you subtract away 12 m.

"10 times the width of the hall"
Call the width of the hall W for simplicity. 10 times the width of the hall will thus be 10*143 m or 1430 m.

"12 m less than 10 times the width of the hall"
Recall from earlier, than if something is 12 m less, one subtracts away 12 m. Combining all of this gives:

L=1430 m - 12 m
Solving this gives you L=1418 m.
Therefore, the length of the hall is 1418 meters.