Some General Information:
⇒ both triangles are isosceles triangle
⇒because two of the angles are equal to each other
⇒ which means there are two equal sides that always join at the
same angle to the base (the third side), and meet directly above
the midpoint of the base (I attached a diagram for a better
explanation)
Question 11:
⇒ x and 10 are equal
⇒ because their sides join the third side at the base with the same
angle
⇒ so x = 10
Question 12:
⇒ '2x + 23' and 7 are equal
⇒ because their sides join the third side at the base with the same
angle
⇒ so:
⇒ thus x = -8
Hope that helps!