Question 58323
Dear A Student,
L1) x-y=7
L2) 3x-11y=-1
L3) x+y+1=0

You found the intersection of L1 and L2 (9.75,2.75)
You found the intersection of L1 and L3 (3,-4) 
Now you have to find the intersection of L2 and L3:
3x-11y=-1
x+y+1=0--->x=-y-1
:
3(-y-1)-11y=-1
-3y-3-11y=-1
-14y-3=-1
-14y=2
{{{-14y/-14=2/-14}}}
{{{y=-1/7}}}
:
{{{x=-(-1/7)-1}}}
{{{x=1/7-1}}}
{{{x=1/7-7/7}}}
{{{x=-6/7}}}
The third vertex is (-6/7,-1/7)
It helps to graph the lines so you can see which lines you need to use for each vertex.
Happy Calculating!!!