We know that x = 5 is the solution of the equation AX-8 = 20 + A. why is the coefficient a here Why not

We know that x = 5 is the solution of the equation AX-8 = 20 + A. why is the coefficient a here Why not

X = 5 is the solution of the equation AX-8 = 20 + A, where is the coefficient a
Now we know the value of X, X is the known value, a becomes an unknown, so the coefficient becomes a