Answer
$8$ years.
Work Step by Step
A difference is the result of subtraction
$\left.\begin{array}{ccc}\text{(effect of eating fruits)}&\text{minus}&\text{ (effect of frequent stress)}\\
3&-&(-5)\end{array}\right.$
$ 3-(-5)=\qquad$subtract $-5$ by adding $5$, its additive opposite
$=3+5$
$=8$
