solve() method

[JorgeyGari]

Add a solve() method that solves for one of the variables.

Examples:

solve(x^(2) - 8x - 9)
    x = -1
    x = 9

solve(y^(2) - 8y - a)
    x = 4-(a+16)^(1/2)
    x = (a+16)^(1/2)+4