Answer: the answer is B
Step-by-step explanation:
The triangle inequality tells you the third side will be between the difference and sum of the other two sides.
5 < third side < 21
3^2 + height^2 = 9^2
9 + height^2 = 81
height ^2 = 72
height = sqrt(72)
Alright,
you will need to solve for each variable to find if they got the equation correctly or not..
Let's start with h
isolate h
(1/2)h = A/(b1+b2)
Now we need to get rid of 1/2 we may do so by multiplying both sides by 2
h = 2(A/(b1+b2))
And that's not how they did it for h so Option D doesn't apply
Let's see C
Also doesn't apply since they multiplied 2 only by A rather than A/h
Let's see B
Also not correct they made the same mistake as with option C
Let's see A
again same mistake
when multiplying both sides by 2
the 2 should be as following

I believe you should contact your instructor and explain that non of the options is right.