First set up your two equations:
x + y = 90
x = 2y - 30
Then substitute what x equals in the second equation into the first equation:
(2y -30) + y = 90
Now solve for y:
3y -30 = 90
3y = 120
y = 40
Then use y = 40 and substitute the value for y into one of your original equations and solve for x. I'll choose the first one, but either one will work.
x+ 40 = 90
x = 50
So your solution is x = 50 and y = 40
The function is

1. let's factorize the expression

:

the zeros of f(x) are the values of x which make f(x) = 0.
from the factorized form of the function, we see that the roots are:
-3, multiplicity 1
3, multiplicity 1
0, multiplicity 3
(the multiplicity of the roots is the power of each factor of f(x) )
2.
The end behavior of f(x), whose term of largest degree is

, is the same as the end behavior of

, which has a well known graph. Check the picture attached.
(similarly the end behavior of an even degree polynomial, could be compared to the end behavior of

)
so, like the graph of

, the graph of

:
"As x goes to negative infinity, f(x) goes to negative infinity, and as x goes to positive infinity, f(x) goes to positive infinity. "
By elimination:
y = 3x - 1
y = 2x + 2
Subtract the second equation from the first
0 = x - 1
y = 2x + 2
Subtract the first equation from the second
0 = x - 1
y = x + 3
Subtract the first equation from the second again
0 = x - 1
y = 4
Subtract x from both sides of the first equation
- x = - 1
y = 4
Divide the first equation by (-1)
x = 1
y = 4
<h3>
So, the solution is x = 1 and y = 4 {or: (1, 4)}</h3>