The given function is,
If the graph of a function f(x) is shifted k units horizontally to the left or to the right, then the new function g(x) is,
If k>0, the graph is shifted to the right.
If k<0, the graph is shifted to the left
Since f(x) is shifted 13 units to the left, then k=-13.
Since f(x)=x, the shifting of f(x) by 13 units to the left gives g(x) as,
Therefore, the equation of the new function g(x)=x+13.