input: Write a function to calculate the slope of a line given two points.
  assert findSlope(5, 9, 15, 40) == 3.1
  assert findSlope(1, 2, 3, 4) == 1.0
  assert findSlope(-1, -2, -3, -4) == 1.0
output: def findSlope(x1, y1, x2, y2):
      return (y2 - y1) / (x2 - x1)