21-7*x>0 inequation

Given the inequality:
$$21 - 7 x > 0$$
To solve this inequality, we must first solve the corresponding equation:
$$21 - 7 x = 0$$
Solve:
Given the linear equation:

21-7*x = 0

Move free summands (without x)
from left part to right part, we given:
$$- 7 x = -21$$
Divide both parts of the equation by -7

x = -21 / (-7)

$$x_{1} = 3$$
$$x_{1} = 3$$
This roots
$$x_{1} = 3$$
is the points with change the sign of the inequality expression.
First define with the sign to the leftmost point:
$$x_{0} < x_{1}$$
For example, let's take the point
$$x_{0} = x_{1} - \frac{1}{10}$$
=
$$- \frac{1}{10} + 3$$
=
$$\frac{29}{10}$$
substitute to the expression
$$21 - 7 x > 0$$
$$21 - \frac{7 \cdot 29}{10} > 0$$

7/10 > 0

the solution of our inequality is:
$$x < 3$$

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