Question 19498
On your first equation there are 2 equal signs.  I'm going to solve this as if it is {{{y=2x-4}}}.  I'll do this step by step so that you can apply the information if the equation is supposed to be something else.
There are 2 methods to solve a system of linear equations, substitution and elimination.  For this system, substitution is easiest, because we know that y equals 2x-4.  Since this information came from the first equation, we use it in the second by substituting all y values in the second equation with 2x-4 to get:
{{{x-4(2x-4)=-7}}} and then solving for x
{{{x-4(2x-4)=-7}}} becomes {{{x-8x-7=-7}}} which becomes {{{-7x=0}}} which becomes {{{x=0}}}
Go back to the first equation and substitute {{{0}}} for all values of x.  This gives you {{{y=2(0)-4}}} which becomes {{{y=-4}}}

I hope this helps!