Notice that, according to the table, when x=0 -> y=1
Moreover, x=3 -> y=7=3*2 + 1
x=5 -> y=11 = 2*5 + 1
It's a straight line!
In the case x=5
y=2*(x=5)+1
Then, the equation of our straight line is: y=2x + 1
We'll say that the rate of change is constant if y=Ax + B, where A and B are numbers or if y=C, C being a number.
(If you want to impress your teacher you can tell him/her that the derivative of the function is constant and therefore it is its rate of change ;) )
Why is the rate of change constant? Because it does not happen that, let's imagine, y=3*x+4 for some value of y or something like that. It always happens that y=2x + 1, those 2 and 1 never change, are constants!