Ax + By = C ,
For your problem:
3x + 4y = 12 is in standard form.
To put it into slope-intercept form, y= mx +b, just solve the equation for y.
y = mx + b, where m is your slope and b is your y intercept.
To go from slope-intercept to standard form, you just need to get x & y on the same side. Do this by subtracting your mx term.