I see the following expression: 3x-4y-12

If a graph is desired then one of the minus symbols should apparently be an equality symbol, as in "3x-4y=12", which is what I suspect was intended.  Otherwise, no simplification for that expression is possible. 

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.