Step-by-step explanation:
The first mower can mow 1/7 of the yard in 1 hour.
The second mower can mow 1/6 of the yard in 1 hour.
Together, they can mow 1/7+1/6+13/42 of the yard in 1 hour.
So it would take them 42/13 hours to mow the entire yard.
42/13=3.2307 hours.
Rounded to the nearest tenth this would be 3.2 hours.