For this we will use equation:
H = starting height + rate_of_growth*periods
We can mark H with index w to represent how many weeks have passed.
Because all of this we can write:
Hw = 200 + 0.5*w
To calculate height after some number of weeks all you need to do is to exchange w with number of weeks.
-5 + n = -15
-15 + 5 = n
-10 = n
Yes it is right that is the right one
1/2x + 1 = 2/3x
1/2x - 2/3x = -1
-x/6 = -1
x = 6
Hope this helps!