Janie is walking in a walk-a-thon to earn money for charity. she walks at a constant rate and for every hour that she walks she makes money. she models her earnings by a linear equation since she is walking at a constant rate. the x-axis shows the amount of time she walks in hours and the y-axis shows the amount of money she earns in tens of dollars. how much will she earn if she walks for four hours? a) $4 b) $8 c) $40 d) $80