The table shows the number of seashells collected by 3 boys.

1. Form an expression for the total number of seashells

$\text{Total} = 17 + x + (6x - 5)$

2. Simplify the algebraic expression

$\text{Total} = x + 6x + 17 - 5 = 7x + 12$

1. Calculate the total number of seashells from the given average

$\text{Total seashells} = 39 \times 3 = 117$

2. Set up and solve the equation for $x$

From part (a), the total is $7x + 12$.
$7x + 12 = 117$
$7x = 117 - 12$
$7x = 105$
$x = \frac{105}{7} = 15$