let h(x) be a compound function of f(x) & g(x)...........................................

The function h(x) will not be differentiable at the point where f(x) and gx) touch each other

For example take f(x) =x and g(x)=2x

so h(x) will take x and 2x values according to the definition

here a =0 as f(x) and g(x) touch ech other and here the function is taking a sharp turn and hence not differentiable