I am having trouble solving this. Need to find the value of a so that the equation will equal $7$ when $x$ goes to infinity.
$$\lim_{x \rightarrow\infty} \left (\sqrt{x^2 + x} - \sqrt{x^2+ax} \right ) = 7$$
What I've tried is to multiply and divide the left side with the conjugate. I've also tried to multiply both sides with the conjugate.