Answer:
When x=0 the friend is 200 miles from the restaurant and at x=5 the friend reaches the restaurant. When x=0 you are 100 miles from the restaurant, so 100 is the y intercept, and when x=5 you reach the restaurant so 5 is the x intercept. We have y=mx+100 and y=0 when x=5, so 0=5m+100 therefore m=-20 and the equation is y=-20x+100.