From what you said;

a + b + 26 = k

b + 26 + c = k

26 + c + d = k

c + d + e = k

d + e + 12 = k

e + 12 + f = k

12 + f + g = k

f + g + h = k

You can start by using

26 + c + d = k

c + d + e = k

d + e + 12 = k

c + d = k - e

Hence

26 + k - e = k

26 - e = 0

And

d + e = k - c

Hence

k - c + 12 = k

12 - c = 0

Solve for c and e and the rest should come in easily.