Ax + By = C is the standard form. Try to see how - 4y = 3 - 2x can fit into Ax + By = C.
Add 2x on both sides to make the x variable present on the other side of the equation.
Now we get
2x - 4y = 3.
A is 2 since that is the number which precedes x.
B is -4 since that is the number which precedes y.
C is 3.
So - 4y = 3 - 2x in standard form would be 2x - 4y = 3.