I'm assuming the equation is:
v = -2000t + 20,000 because the value of a car decreases with time
v = worth of the car
t = number of years
Since you know the value of v = 10,000, substitute or plug in 10,000 for v in order to find t
v = -2000t + 20,000 Substitute 10,000 into v [To find t, you need to get the variable by itself to one side of the equation.]
10,000 = -2000t + 20,000 Subtract 20,000 on both sides
-10,000 = -2000t Divide -2000 on both sides to get t by itself [negative divided by a negative results in a positive]
5 = t
