The wall is 9 ft by 12 ft.
9 × 12 = 108
The area of the wall is 108 ft².
Not sure what the width of the wallpaper is for <:(
The equation of the line in point-slope form is given by:

Where,
(xo, yo): point that belongs to the line
m: slope of the line
The slope is given by:

Substituting values:

Rewriting:

Now we choose an ordered pair:

Substituting values we have:

We now rewrite the equation in its standard form:

Where,
b: cut with the y axis.
We have then:
Answer: point-slope form: 
or
standard form
Are you looking for the value of x?
We can complete the square/factor:
x^2+7x-30=0
(x+10)(x-3)=0
By the zero product property rule, we have two equations, x+10=0 and x-3=0.
So the zeroes of x are -10 and 3
Hope this helped. Comment below if this didn't make sense.
12 cubic feet!! this may be it
I believe your answer would be 14(6+3x) XD