this works because of the addition property of equality
a=16--->a+3=16+3
reverse engineering
a+3=16+3--->a=16
now given that 5x-4y=-30
and given also the equation 3x+4y=38
by the addition property
3x+4y+(-30)=38+(-30)
since -30=5x-4y by substituting
3x+4y+(5x-4y)=38-30
which yields
8x=8
the addition property of equality which is B.