We need to find an expression which will help Sara calculate the total weight of the truck.
By hypothesis, we know that x represents the number of cans in the truck. And each can weigh 14 ounces, so the total weight of the cans in the truck is 14*x = 14x.
Now in the truck, there's a total of 840 cans and bottles combined. And x is the number of cans. So 840 - x is the number of bottles. And since each bottle's weight is 70 ounces, the total weight of the bottles in the truck is 70*(840-x) = 70*840 - 70*x = 58800 - 70x.
Now to find the total weight, we add the weight of the cans to the weight of the bottle: 14x + 58800 - 70x = 58800-56x.
So the equation to find the total weight of the truck is:
58800-56x, in which x represents the number of cans in the truck.
Hope this helps! :)