A+b=18

b+c=15

d-c=18

d-a=15

Solve for a, b, c, and d

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