15-3x<=0 inequation

Given the inequality:
$$15 - 3 x \leq 0$$
To solve this inequality, we must first solve the corresponding equation:
$$15 - 3 x = 0$$
Solve:
Given the linear equation:

15-3*x = 0

Move free summands (without x)
from left part to right part, we given:
$$- 3 x = -15$$
Divide both parts of the equation by -3

x = -15 / (-3)

$$x_{1} = 5$$
$$x_{1} = 5$$
This roots
$$x_{1} = 5$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} \leq x_{1}$$
For example, let's take the point
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + 5$$
=
$$\frac{49}{10}$$
substitute to the expression
$$15 - 3 x \leq 0$$
$$15 - \frac{3 \cdot 49}{10} \leq 0$$

3/10 <= 0

but

3/10 >= 0

Then
$$x \leq 5$$
no execute
the solution of our inequality is:
$$x \geq 5$$

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