Clarification for solve function

This is the code I have written:

x = 0:1:15;

U = 2*x+5;

L = 3*x-10;

u = 2*(x-1)+5;

l = 3*(x-1)-10;

U_plus = 2*(x+1)+5;

L_plus = 3*(x+1)-10;

c = abs(U_plus - L_plus);

C = abs(U-L);

C_plus = abs(u-l);

s1 = solve(abs(U_plus-L_plus)==0,x);

s2 = solve(abs(U-L)==0,x);

s3 = solve(abs(u-l)==0,x);

This is the error I am facing:

Error using sym.getEqnsVars>checkVariables (line 87)

Second argument must be a vector of symbolic variables.

Error in sym.getEqnsVars (line 54)

checkVariables(vars);

Error in solve>getEqns (line 429)

[eqns, vars] = sym.getEqnsVars(argv{:});

Error in solve (line 226)

[eqns,vars,options] = getEqns(varargin{:});

Error in Final (line 21)

s1 = solve(abs(U_plus-L_plus)==0,x);

Am unable to debug this. Any help is highly appreciated

Answer 1

[KALYAN ACHARJYA]

syms x;
U = 2*x+5;
L = 3*x-10;
u = 2*(x-1)+5;
l = 3*(x-1)-10;
U_plus = 2*(x+1)+5;
L_plus = 3*(x+1)-10;
c = abs(U_plus - L_plus);
C = abs(U-L);
C_plus = abs(u-l);
s1 = solve(abs(U_plus-L_plus)==0,x)
s2 = solve(abs(U-L)==0,x)
s3 = solve(abs(u-l)==0,x)