By "-21x + 9 + y equals -24x + 8" I'm assuming you meant
"y = -21x + 9 AND y equals -24x + 8." Using " + " to indicate "and" could be confusing.
We then have the system of linear equations
y = -21x + 9
y = -24x + 8
The function with the greater slope is the 2nd: y = -24x + 8 (with the understanding that we're focusing on the magnitude of the slopes). Thus, it is this function that changes more quickly.