The purple color in a purple pea plant is dominant to a white-color. what would be the expected outcome of a cross between a purple-flowered pea plant, and a white-flowered pea plant?

Well, it depends on the purple pea plant. Purple pea plants can have either PP or Pp.

White pea plants always have recessive, so it would have pp only.

So, the expected outcome of the cross would be - 
PP x pp or Pp x pp

PP x pp = all the offspring would be purple in colour 
 or 
Pp x pp = half would be purple and the other half would be white

If you want the answer you need to post the figure above that it's talking about and you