now, let's notice those tickmarks on the sides.
the ones at the bottom, those two sides are equal.
the ones at the top-right, those two sides are also equal.
what does all that mean?
it means that, the segment in the triangle, the 2x-3 segment is really the midsegment of the triangle, and therefore, also parallel to the base of 18, and also therefore is half of that length.
[tex]\bf \textit{midsegment of a triangle}\\\\ m = \cfrac{b}{2}~~ \begin{cases} b=parallel~side\\ \qquad or~base\\[-0.5em] \hrulefill\\ m=2x-3\\ b=18 \end{cases}\implies 2x-3=\cfrac{18}{2}\implies 2x-3=9 \\\\\\ 2x=12\implies x=\cfrac{12}{2}\implies x=6[/tex]
