Chain rule for exponential functions

I have a question I could use a little help with here...

Find the equation of the tangent to the function f(x) = 3^(x^2+1) - x^2 at (1 , 8)

I have used the difference rulwe and the chain rule for exp. functions to come up with the derivative:

2x(3^(x^2+1)) (ln3) - 2x

So using 1 for the value of x I come up with m = 17.77

I have the solution to the question which shows one of the last few steps of the solution as:

y = 17.77(x-1) + 8

If the formula for the equation is y = mx + b.... Why does the solution have (x - 1) as the x value. And how did they come up with 8 as the y - intercept? Thanks!