Question

Also this question tooo pleease, PS: what topic is this Solve for x.

Answer 1

This falls under geometry. The two labeled angles are congruent, which means the line segment between them (AD in the image) is a bisector of the angle at vertex A. Use the angle bisector theorem, which says that

$\dfrac{BD}{DC}=\dfrac{AB}{AC}\iff\dfrac{12}{(5+9x)-12}=\dfrac{15}{25}$

$\dfrac{12}{9x-7}=\dfrac35$

$20=9x-7$

$x=3$