What are the x-coordinates of the solutions to this system of equations?
x2 + y2 = 16
y = x + 4

What this question is asking is at what points do these function/relation intersect. You can solve this by graphing it, or solve it algebraically.

You are given x^2 + y^2 = 16 and y = x + 4

You can substitute y = x + 4 into x^2 + y^2 = 16
So we get

x^2 + (x+4)^2 = 16

This simplifies into

x^2 + x^2 + 8x + 16 = 16

Combine like terms

2x^2 + 8x = 0

Then factor 2x out:

2x( x + 4 ) = 0

We can see that our solution is x = 0, x = -4

So our answer is 

x = 0, x = -4
Hopes this helps!