How do you find the x-intercepts of a quadratic function in standard form given #f(x) = 2(x+1)^2 - 3#?

Redirected from "Suppose that I don't have a formula for #g(x)# but I know that #g(1) = 3# and #g'(x) = sqrt(x^2+15)# for all x. How do I use a linear approximation to estimate #g(0.9)# and #g(1.1)#?"