Basic procedure:

Set up a spreadsheet to calculate f(x) at some initial guess for x. You will have one cell containing x, and one cell containing the formula for f(x).
Then call Solver and tell it to set f(x) to a value of 7 by changing x. If you have given it a suitable starting value, it will find what value of x results in f(x)=7

I would probably suggest you review how the newton raphson algorithm works (a quick internet search for newton's method or similar should yield several tutorials) so you will understand how the algorithm works, including important considerations for how to choose a suitable initial guess.