How do you simplify the expression 7x2−4y+3x2+5y+2 and evaluate it for x+3and y=9?

${\displaystyle y=-10x^2-2}$
${\displaystyle ?x=3-y=-92}$
${\displaystyle ?y=x+3}$- No real solution
${\displaystyle y=9}$- No real solutions
Explanation:
Assuming this is all equal to 0, we add like terms
${\displaystyle 10x^2+y+2=0}$
We can then isolate one of the variables, usually y.
${\displaystyle y=-10x^2-2}$
To evaluate it for ${\displaystyle x=3}$, just plug it into the function to get
${\displaystyle y\left(3\right)=-10{\left(3\right)}^2-2=-10\left(9\right)-2=-90-2=-92}$
To solve for ${\displaystyle y=x+3}$ we get
${\displaystyle x+3=-10x^2-2}$
${\displaystyle 10x^2+x+5=0}$
We can use the quadratic formula to get the solutions
${\displaystyle \frac{-{\left(1\right)}^2\pm \sqrt{{\left(1\right)}^2-4\left(10\right)\left(5\right)}}{2\left(10\right)}}$
${\displaystyle \frac{-1\pm \sqrt{1-200}}{20}}$
${\displaystyle \frac{-1\pm \sqrt{-199}}{20}}$
This results in undefined so there are no real solutions.
For ${\displaystyle y=9}$ we do the following
${\displaystyle 9=-10x^2-2}$
${\displaystyle -10x^2=11}$
${\displaystyle x^2=\frac{11}{-10}}$
${\displaystyle x=\sqrt{-\frac{11}{10}}}$
This is also undefined, so there are no real solutions.