b$)$ Consider the piecewise$-$defined function:

$f(x,y)=\{\begin{array}{ccc}\frac{e^{-\frac{1}{x^2+y^2}}}{x^2+y^2},& (x,y)(0,& 0)\\ a,& (x,y)=(0,& 0)\end{array}$

where $a$ is a constant.

Task: Determine the value of a that makes the function $f(x,y)$ continuous at $(0,0).$