a+b= 18
Move b to the other side. Sign changes from+b to -b.
a+b-b= 18-b
a= 18-b or a= -b+18
b+c=15
Move c to the other side. Sign changes from +c to -c.
b+c-c= 15-c
b= 15-c or b=-c+15
d-c=18
Move d to the other side. Sign changes from +d to -d.
d-d-c= 18-d
-c= 18-d ( Multiply by -1)
(-1)(-c)= -1(18)-1(-d)
c= -18+d or c= d-18
d-a= 15
Move -a to the other side. Sign changes from -a to a.
d-a+a= 15+a
d= 15+a or d= a+15
Answers:
a+b=18: a= 18-b or a= -b+18
b+c=15: b= 15-c or b=-c+15
d-c=18: c= -18+d or c= d-18
d-a= 15: d= 15+a or d= a+15
