One equation, two variables, choose which variable to solve for... 'y' was chosen, hence algebraic manipulations are applied so that 'y' is left alone on the left side and the result is on the right side.  That form is typical of linear equations but can be clearly stated when graphed (click the 'Graph' tab and enter the equation for graphing). 

Click to see all the steps

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.