How to get the solution of this simple optimization problem?

$\displaystyle

minimize~~\Phi = x_1^2 + x_2^2

$

$\displaystyle

subject~to~~x_1 +2x_2 = 5

$

I konw we may use Moore-Penrose inverse like,

$\displaystyle

Ax = b => x = (A^TA)^{-1}A^Tb

$

but what are $\displaystyle A$ and $\displaystyle x$ now?

if $\displaystyle A = (1~~2) $ and $\displaystyle A^T = (1~~2)^T $ then there is no inverse of $\displaystyle A^TA$. I am quite confused how to formulate these $\displaystyle A$ and $\displaystyle x$ to do the matrix multiplication.

Thanks a lot