Question 149711
These are called "systems of equations" when you have 
several equations with a certain number of unknowns.
It is an absolute fact that the number of equations
has to match the number of unknowns in order to find
values (numbers) for all the unknowns. 
You have 3 equations and 4 unknowns, so you need 1 more
equation to find what x,y,a, and b are. What you can
find is x and y in terms of a and b. That would only
be 2 unknowns. I'm not sure if that is what's being asked for.
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The other big thing to what for is systems of 
equations that don't give you answers at all, like
(1){{{y = 4x}}}
(2){{{2y = 8x}}}
(3){{{4y = 16x}}}
These are the same equation repeated 3 times
Divide both sides of (2) by 2 and you get (1)
Divide both sides of (3) by 4 and you also get (1)
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Also there are systems that just have no intersections
and can't be solved, like
{{{y = -x}}}
{{{y = x^2 - 10x + 25}}}
I'll plot these:
{{{ graph( 600, 600, -20, 20, -20, 20,-x, x^2 - 10x + 25) }}}