See the attached figure
A(4,8) B(0,7) C(4,3)
<span>the line of reflection is x=5
</span>
the reflection is (x,y)-------------------> (10-x,y)
therefore
A(4,8)-----------------> A1(10-4,8)------> A1(6,8)
B(0,7)-----------------> B1(10-0,7)------> B1(10,7)
C(4,3)-----------------> C1(10-4,3)------> C1(6,3)
