3. $\frac{4}{n + 9}=\frac{2}{7}$

Explanation:

Step1: Cross - multiply

$4\times7 = 2\times(n + 9)$

Step2: Expand the right - hand side

$28=2n + 18$

Step3: Subtract 18 from both sides

$28 - 18=2n+18 - 18$ $10 = 2n$

Step4: Divide both sides by 2

$\frac{10}{2}=\frac{2n}{2}$ $n = 5$

Answer:

$n = 5$