### Explanation

**Multiplication and substitution** is a technique often used in algebra, particularly for solving systems of linear equations and manipulating algebraic expressions.

### Multiplication

Multiplication involves scaling an equation or an expression by a constant factor. Here, the idea is to **multiply both sides of an equation** by the same non-zero constant to simplify or adjust coefficients. For example, consider the equation:

$ax + by = c$
Multiplying both sides by a constant $k$ gives:

$k(ax + by) = kc$
### Substitution

Substitution involves replacing a variable with an equivalent expression from another equation. This method is especially useful when dealing with systems of equations. Consider the system of equations:

- $x + y = 5$
- $2x - y = 3$

First, we can solve the first equation for $y$:

$y = 5 - x$
Next, we **substitute** this expression for $y$ in the second equation:

$2x - (5 - x) = 3$
Simplifying this equation:

$2x - 5 + x = 3$
$3x - 5 = 3$
$3x = 8$
$x = \frac{8}{3}$
Then, we substitute back to find $y$:

$y = 5 - \frac{8}{3}$
$y = \frac{15}{3} - \frac{8}{3}$
$y = \frac{7}{3}$
Thus, the **solution** to the system is:

$x = \frac{8}{3}, \quad y = \frac{7}{3}$
### Key Points

**Multiplication** is used to adjust the coefficients of equations.
**Substitution** is used to eliminate variables by replacing them with expressions from other equations.
- These methods are often applied in tandem to simplify and solve systems of equations.