Answer:
Step-by-step explanation:
It is important to remember that, by definition, the measure of an exterior angle of a triangle is equal to the sum of the measures of the two non-adjacent interior angles.
In this case we can idenfity that the angle 
is an exterior angle of the triangle QRT. Then:
Where:
Substituting values and solving for "x", we get:
Then:
The sum of the interior angles of a triangle is 180°, therefore:
Solving for 
, we get:
When two shapes are mathematically similar, that means that all their sides are in a proportion.
We know the sieds of EH and AD, and so we can find the proportion of the shapes using these two sides.
EH/AD = 6/9 = 2/3
The ratio of EFGH to ABCD is 2:3
Now we have to find BC.
Becuase they're in proportion, we can form this equation:
= 
Cross multiply:
24 = 2*BC
Divide both sides by 2:
BC = 12 cm
Good luck!
