P=s1+s2+s3
Move +s1 to the other side. Sign changes from +s1 to -s1
P-s1=s1-s1+s2+s3
p-s1=s2+s3
Move s2 to the other side. Sign changes from s2 to -s2
p-s1-s2=s2-s2+s3
p-s1-s2=s3
Answer: p-s1-s2=s3
