Jack usually mows his lawn in 6 hours means his rate is 1/6 lawns per hour

Marilyn can mow the same yard in 7 hours means his rate is 1/7 lawns per hour

let x represent the hours they need to mow the lawn together thus their together rate is 1/x lawns per hour

1/6 + 1/7 = 1/x

by solving we find

x = 42/13

x = 3 hr 3*60/13 min

x = 3 hr 13.8 min

click here to see the step by step solution of the equation

it will take them 3 hr 13.8 min to mow the lawn together.