Filling in the blanks:

.a.b.26.c.d.e.12.f.g.h.

Resulting equations:

a+b+26 = x

b+26+c=x

26+c+d=x

c+d+e=x

d+e+12=x

e+12+f=x

12+f+g=x

f+g+h=x

a+b+26+c+d+e+12+f+g+h = 210

You have nine equations and nine unknowns.

There are several ways to attack this:

You can remove the x by subtracting the adjoining pairs.

a+b+26 = x

b+26+c=x

a - c = 0

26+c+d=x

c+d+e=x

26 - e =0

d+e+12=x

e+12+f=x

d - f =0

12+f+g=x

f+g+h=x

12 - h = 0

That's a start.

and continue with every other equation

a+b+26 = x

26+c+d=x

a+b = c+d

26+c+d=x

d+e+12=x

26+c = e +12

etc..

Or create an augmented matrix and reduce to a triangular matrix.