Answer
$9$ years.
Work Step by Step
A difference is the result of subtraction.
$\left.\begin{array}{ccc}\text{(effect of lacking education)}&\text{minus}&\text{ (effect of smoking)}\\
-6&-&(-15)\end{array}\right.$
$-6-(-15)\qquad$subtract $-15$ by adding $15$, its additive opposite
$=-6+15\qquad$ use the rule for adding numbers of different signs
$=+9$
