Answer:

Δn=3
Step-by-step explanation:
Remember, if we need to divide the interval (a,b) in n equal subinterval, then we need divide the distance (d) between the endpoints of the interval and divide it by n. Then the width Δn of each subinterval is d/n.
We have the interval [-5,7]. The distance between the endpoints of the interval is
.
Now, we divide d by 4 and obtain 
Then, Δn=3.
Now, to find the endpoints of each sub-interval, we add 3 from the left end of the interval.

So,
