Hello, could you please show how to prove this.

On problems 27 - 29, use the definition of continuity to prove that the function is discontinuous at the given value of a. Sketch the graph of the function. 27. f(x) = x2-5x+4 ,a=1 x2-3x x-1 28. g (x) = x2-9 if x # 3 Sex if x o' a =0 1 if x = 3