Okay, it's wrong because they forgot to do -2 times 3x.
The correct answer is

-2x-8
Hope i got is correct!
Add the two angles and then subtract from 180* to find the missing angle
So,
145* + 15* = 160*
180* - 160* = 20*
Answer: x = 20*
To start this problem, what we need to do is try to simplify the fraction into one we might know already. The best way to go about doing this is to see if we can divide the top and bottom by any of the same numbers.
We can divide 9 by 1, 3, and 9
We can divide 12 by 1, 3, 4, and 12
Looking back over the numbers, we can see that both 9 and 12 are divisible by 3, so if we divide both of them by 3, we will get a new fraction! This fraction will be 3/4.
Our next step is to see if we know what 3/4 is. I know 1/4 is .25 because the coin called a quarter is 25 cents, so if I had 3 quarters, I would have 75 cents, so I know know that 9/12 = 3/4 = .75!
The general formula for the lateral surface area of a regular pyramid is:

Where:
p=perimeter of a base
l=slant height
a.
a=3ft
l=4ft
p=5*a=5*3=15ft

b.
a=2.5ft
l=4ft
p=6*a=6*2.5=15ft

c.
a=1.9ft
l=4ft
p=8*a=8*1.9=15.2ft

d.
Based on a lateral surface area the blue umbrella is best choice. But, the best umbrella overall is the one with the greatest base area. General formula for regular polygon is:

For pentagon:
A=15.48ft^2
For hexagon:
A=16.24ft^2
For octagon:
A=17.43ft^2
Blue umbrella is still best choice.