Question 49925
The line of symnmetry will be at the vertex.  The vertex for y = ax^2 + bx +c will always be at {{{x=(-b)/(2a) }}}


In this case a=2 and b = -8, so vertex is at {{{x=8/(2*2)=2}}}


To find the x-intercepts, let y = 0, and 
solve the equation 0=2x^2 - 8x - 42 by factoring if possible.


2(x^2 - 4x -21)=0
2(x-7)(x+3)=0
x= 7 ;  x = -3


This is a parabola that opens upward.  You didn't ask to graph it, but it should look like this:
{{{graph (300,300,-10,10,-50,46, 2x^2-8x-42) }}}


R^2 at SCC