float slope = (readings-readings)/(readings-readings);
The readings values are all stored as ints and unless you tell the compiler to use them differently the math done on the right hand side of the = will be done as ints. You tell the compiler how by using a cast
and remember that inside of parens is a little domain where casts outside only get applied after being operated on.
int A = 1;
int B = 5;
Serial.println( A/B ); // prints 0
C = (float) A / (float) B; // C now == 0.2 or very close, floats are "funny" that way.
(float) above is the cast. Verb of cast is casting. "Casting" is short for "type casting".
Mixing data types without specifying how they are to interact is the cause of lots of hair loss and not just for newbs.