I was given the answer but don’t really understand how it got to that answer—please explain!

Given:
The line is reflected across the y-axis.
The triangle is formed that is enclosed by two lines and the x-axis.
The triangle is,
In the above triangle, the base is 6+6=12 units. and heights is 3 unit.
[tex]\begin{gathered} A(\text{triangle)=}\frac{1}{2}\times height\times base \\ =\frac{1}{2}\times3\times12 \\ =18 \end{gathered}[/tex]Answer: 18 square unit.