Solve the following using system of equations:
" x2 + y2 = 13
2x - y = 4
"

One of the things you can do here is rearrange the equations like this: 
x2 +y2 -13 = 0 
y = 2x + 4 
After that what you can do is substitute: 
x2 + (2x + 4)2 -13 = 0 
x2 + (4x2 +16x + 16) -13 = 0 
5x2 +16x + 3 = 0 
x = (-16 + sqrt(16^2 - 4*5*3))/(2*5) and (-16 - sqrt(16^2 - 4*5*3))/(2*5) 
x = (-16 + 14)/10 and (-16 - 14)/10 
x = -0.2 and -3 

y = 2x + 4 
y = 2*(-0.2) + 4 = 3.6 
y = 2*(-3) + 4 = -2 
The answer will be 
(-0.2,3.6) and (-3,-2)
Hope this helps you greatly