It would be y=3x+5. This is because if it is going at a rate of 3cm a month, this makes it the constant value or the slope. But since it’s starting value is 5cm, that would make it the y-intercept, which is why your answer would be y=3x+5.
The plant starts at 5cm so +5 is the starting height, It grows at 3cm a month making 3x the growth rate. so 3x+5 is the height of the plant every month.
