Wow... what grade are you even in to be getting a problem like that? that's a difficult one...
the x and y values makes both equations true
Explanation
a linear system is the set of 2 linear equations.
Step 1
the solution to a linear system is the coordiante where the line intersect, this coordiante satisfies both equations, then
the x and y values makes both equations true
If you're trying to find the missing side of the "triangle" (the distance from the house to where the ladder touches the ground), you would use the pythagorean theorem, a²+b²=c². (c is always hypotenuse)
a²+b²=c²
12²+b²=15²
144+b²=225
b²=81
√b²=√81
b=9