MATLAB: How to solve equation without values

Question

hi,

I just want to solve the following equation. Just the simplified form without values.

I mean I just want to do the following task:

X1= 2- 4a

X2=3- 4a

I just want to use these values in the following expression

and get the simplified equation with only "a" as the variable

Answer 1

syms X1 X2 a
expr = 4*X1^2 - 4*X1*X2 + 2*X2^2;
new_expr = subs(expr, {X1, X2}, {2-4*a, 3-4*a});
simplify(new_expr)