Draw a tree diagram!

Pr(E) = 0.1 and Pr(E') = 0.9 are the first two branches.

From E the two branches are Pr(+ve) = 0.97 and Pr(-ve) = 0.03. (Note that these probabilities are actually Pr(+ve | E) = 0.97 and Pr(-ve | E) = 0.03).

From E' the two branches are Pr(+ve) = 0.02 and Pr(-ve) = 0.98. (Note that these probabilities are actually Pr(+ve | E') = 0.02 and Pr(-ve | E') = 0.98).

Then Pr(E | +ve) =