the standard formula is y= mx + b
your equation is 4x + 3y = -21 so first lets get the y variable by itself and remove the x variable by canceling it out, meaning we are going to subtract
4x + 3y = -21
-4x -4x
3y = -4x - 21 and now we are going to get the y by itself by dividing by 3
÷3 ÷3 ÷3
y = 1 1/3x -7 that is your answer i hope this helps