The x remains the same while the y changes sign. So if it’s (1,4) the reflection will be (1,-4). Hope fully that helps.
        
             
        
        
        
We can just substitute y = 4x+1 in the second equation.

We know the value of x, substitute x in any equations to get the y-value.

The value of y is 3 and the value of x is 0.5 aka 1/2
Since you want the solution as a point then It's below here.
 
        
             
        
        
        
1/2x + 5 = 35
You use 1/2 because she sold half of her books . Then +5 because she gained 5 back and 35 because your end total should be 35. 
Hope this helps ! :)