How to find the standard form of 9+3y=-15x?

Standard form is Ax+By=C

Simply rearrange the terms by adding and subtracting. Then, divide all terms by any common factors.

9+3y-=-15x (add 15x and subtract 9 from both sides)

15x+3y=-9 (all terms contain a 3, so divide it from all of them

5x+y=-3