if i recall correctly, the vertical asymptote would become a horizontal asymptote.
e.g if the original function as x = 1 as the asymptote, then the inverse function would have an asymptote at y = 1
I'm assuming you mean
\int x (2x + 7) \ dx
your step is correct so far,regarding the bolded part it doesn't have to be.
u = 2x+7 \Rightarrow \ x = \frac{u-7}{2} \\ \\ \therefore \ \int x (2x + 7) \ dx = \int (\frac{1}{2})(\frac{u-7}{2})(u) \ du
it means there's an infinite number of solutions rather than 1 unique solution. you just need to find the conditions of the solution.
Anyways the row echelon form is
1 4 1
0 1 1
0 0 -4
Let x_3 = k \\ \\ \therefore x_2+x_3 = x_2 + k = 0 \Rightarrow \ \ x_2 = -k \\ \\ x_1+4x_2+x_3 = 0...