Curve equations

Did this in class, ended up giving up. Friend told me the answer but i still dont know what the method would be to do it..

A curve has the equation y=9x-4x^3
Points A and B are on the curve and have the x coordinates -2 and 1 respectively.
show that the length AB is k√10 where k is a constant to be found...

I have no Idea where to start with this, so any help would be brilliant! thanks

sub each of the x coordinates to get the corresponding y coordinates

the use the distance formula

Sorry, but whay do you mean by the distance formula? Ive never heard of it before.

How have you been taught how to find the distance between 2 points given their coordinates?

root of (x2-x1)^2+(y2-y1)^2

yes

Sorry! I though you meant something else thanks for help!