Growth Rate of the Plant in a Science Experiment

During a science experiment, a group of students recorded the growth of a plant over a week. On the first day, the height of the plant was represented by the variable $$h$$. Each day, the plant grows by a certain amount represented by $$g$$. By the end of the week (7 days), the plant's height can be modeled by the equation:

$$h + 7g = 35$$

On day 3, the height of the plant was recorded as 21 inches. This leads to the equation:

$$h + 3g = 21$$

What is the growth of the plant each day (i.e., the value of $$g$$)?