I'm assuming that you mean
because if you meant
then u would simplify and you couldn't make it the subject.
Under this assumption, we start with
We multiply both sides by 
We expand the left hand side:
We move all terms involving u to the left and all terms not involving u to the right:
We factor u on the left hand side:
We divide both sides by 
9 because 1,800 divided by 200 is 9
The answer you have is good. You don't need to change anything.
Answer:
a) x = 2
b) x = 4
Step-by-step explanation:
a = (b₁ + b₂)/2 * h
----------------------------
a)
48 = (10 + x)/2 * 8
Divide both sides by 8
6 = (10 + x)/2
multiply both sides by 2
12 = 10 + x
Subtract 10 from both sides
2 = x
x = 2
---------------------------------
b)
18 = (7 + 2)/2 * x
18 = 4.5 * x
divide both sides by 4.5
4 = x
x = 4
