I'm currently going through some questions in my textbook and came across a question on a quadratic and linear simultaneous equation.

Solve the equation:

x + y = 1

x^2 + y^2 = 16

I am not interested in the question itself but rather the explanation the book gives me which says after a couple of steps shows the equation:

2x^2 - 2x -15 = 0

Then it tells me to divide the equation by 2 and says when you divide the equation by 2 you will get:

x^2 - x - 15 = 0

I'm confused because i thought the -15 would also have to divide by two...

is it the book or am i missing something serious here?