You will need
  • Math.
Write the equation of a straight line and Express the ordinate of a function through the x point. For example, let the equation of the line: 3x + 4y = 13. Express the y coordinate: y = -3x/4 + 13/4.
The coefficient in front of x will be the slope of a straight line in the Cartesian coordinate system. That is, the slope of k=-3/4.
To find the angle between line and x-axis, it is sufficient to calculate the arc tangent of the angular coefficient. Thus the angle between line 3x + 4y = 13 and x-axis is equal to: U = artg(-3/4) = -36 degrees.