Y=-x^2-4x+23 and 2x+y=15 what is the solution

This is a system of equations, meaning you find the value of one variable and substitute it into the other equation
if y=-x^2-4x+23 we can put that into the second equation for y
This gives us 2x+(-x^2-4x+23)=15
Lets simplify this and set it equal to 0 by moving the 15 over
-x^2-2x+8=0
This gets us x=-4, 2
Lets try plugging both those back into our first equation and see what value of y we get
y=-(-4^2)-4(-4)+23; y=23
y=-(2^2)-4(2)+23; y=11
So you get two sets of solutions
(-4,23) and (2,11)