Question 172259
Interesting!
First, if you were to substitute the answers brought home by your niece, you would quickly see that they do not work in the given equation:
{{{5.5x-1.5y = 3.5}}} Substituting x = 3 and y = 4, you get:
{{{5.5(3)-1.5(4) = 3.5}}} Simplifying...
{{{16.5-6 = 3.5}}}
{{{10.6 <> 3.5}}}
Second, in your attempt to simplify the given equation, you cannot subtract 3.5 (a constant term) from 5.5x (a variable term) to get 2x.  Why, that's like trying to subtract 3 pesos from 5 dollars...it can't be done directly!
Is it possible that your niece inadvertantly dropped the other equation on her way home from school?
Now there is a class of equations in which you have more unknowns than you have equations, and these are called Diophantine equations.  But their solutions depend on certain restrictions placed on the variables, e.g. they must be integers.