n = 11;
x   = { x1, x2, x3, x4, x5, x6, x7, x8, x9, x10, x11 };
dx  = Simplify[ x[[ Range[2,n] ]] -  x[[ Range[1,n-1] ]] ];
ddx = Simplify[dx[[ Range[2,n-1] ]] - dx[[ Range[1,n-2] ]] ];

m = 4;
ddz = ddx; ddz[[m-2]] = a; ddz[[m-1]] = b;	
dz = dx;
Do[ dz[[i]]=dz[[i-1]]+ddz[[i-1]], {i,2,n-1} ];
dz  = Simplify[dz];
z=x;
Do[  z[[i]]= z[[i-1]]+ dz[[i-1]], {i,2,n} ];
 z  = Simplify[ z];

eq1 = z[[m]] == x[[m]] - H;
eq2 = z[[m+1]] == x[[m+1]] - H;

sol = Simplify[ Solve[ {eq1,eq2}, {a,H} ]];

	a -> (x2 - x3 - x4 + x5)/2, 
	H -> (x2 - 3*x3 + 3*x4 - x5)/2
This is only the best approximation but it is not necesserily accurate!