Are you sure the scaling is correct? It's just that it's odd to have metres and feet (Imperial System and SI) in the same question.
Ok, assuming you mean 30 feet by 35 feet,
Then the area of the lawn is square feet
Now, since 5 pounds covers 750 square feet, 50 pounds (1 bag) will cover 7500 square feet.
Now we can use ratios to determine how many bags we need. Let x be the number of bags we need.
As you can see, the ratios of the areas of the lawn is
and the ratio of the bags needed is
Now simply solve for x:
of a bag.