If you have 2 unknow variables you must have 2 equations.
That is impossible to solve.
X=823
y=-212.25
To solve the equation 0.1X + 0.4Y = -2.6 using the substitution method, we need another equation in terms of either X or Y. Without another equation, we cannot find the exact values of X and Y.
If you have a second equation, please provide it so we can proceed with the substitution method.