Suppose you're climbing a mountain. At the beginning you're 25 meters below a reference height, then your position is -25 m. If you climb at a rate of 2 meters per minute, after x minutes your height will be 2x above your starting position. Then, your height will be 2x - 25, after x minutes. If you want to reach 75 meters above the reference, then to find how many minutes do you need, you have to solve the next equation 2x - 25 = 75.