We know that this relationship is linear so we will be using the equation of a line which takes the form y = mx + b where m is the slope and b is the y-intercept.
Our y variable will be the amount of times the dragonfly has beat its wings and the x variable will be the time.
We know that our slope will relate these and it is given that beats its wings 30 times / second. This is good because when we multiply it by seconds, the seconds unit will cancel, leaving us with an amount of beats.
To find b, let's consider how many times the dragonfly has beat its wings at time 0s. When we think about this, we realize that the number is 0 so our y-intercept is also 0.
Thus, the equation of the line is y = 30x
