Hello!
You can only add variables with the same base and exponent
8u^3 + 4u^3 = 12u^3
8u^2 + 0 = 8u^2
0 + -6u = -6u
6 + 3 = 9
Put the sums together
12u^3 + 8u^2 - 6u + 9
The answer is

Hope this helps!
-7x + y = -6 |add 7x to both sides
y = 7x - 6
the slope: m = 7
the y-intercept: y = -6
2(15+N)
replace a number by N
replace the sum of 15 and N by (15+N)
and lastly, replace twice by 2
So your answer is 2(15+N)
3, if you do the y^2-y^1/x^2-x^1 you will get 3/1 which will be simplified to 3.