For any graphable equation, the software first manipulates it into its standard form before graphing; for a linear equation, that form is "y=mx+b" where the slope "m" may then be determined by inspecting the equation (for example, in the equation "y=-2x+4" "-2" is the correct value for "m"). For a parabolic equation, the standard form is y = a(x - h)^2 + k, from which direction (polarity of "a") and axis of symmetry (value of "h"), etc. may be determined by inspection.

The best way to solve a myriad of line equation is to use Algebrator.  The attached images show the approach applied to doyourmath.com through Algebrator.  The slope is then 16/7.