Write a system of equations to describe the situation below, solve using substitution, and fill in the blanks. Erin is collecting pledges for a walk-a-thon. Her mother has pledged a flat donation of $7, and her grandmother has pledged $1 per kilometer. If Erin walks a certain distance, the two donors will end up owing the same amount. What is that distance? How much will each donor owe?

If Erin walks kilometers, her mother and grandmother will each owe $