0 it has no simplest form
One way to see if two ratios are proportional is to write them as fractions and then reduce them. If the reduced fractions are the same, your ratios are proportional.
The base is 5 because when you divide 31.5 by 6.3 it's 5
-2x/5-2=18
-2x/5-2+2=18+2
-2x/5=20
-2x/5x5=20x5
-2x=100
-2x/-2=100/-2
x=-50
