standard form is x + y = number
The equation has a slope of 3 and a y intercept of -4 so the equation should be
y = 3x - 4
To get this in standard form you have to move the 3x to the other side.
y - 3x = -4
-3x + y = -4
The coefficient before x cannot be negative in standard form so you have to multiply the whole equation by -1.
3x - y = 4