Don't understand a question on Euler's method on this site's quiz

I was working on a question on this site about the said theme. I ran into a question that I failed to answer correctly. I looked at the answer and could not comprehend. See the pic below:

dy/dx: 2x+y and y(1)=k

Using Euler’s method, starting at x=1  with a step-size of 0.5 , gives the approximation y(1.5)=4. 

Find k.

The answer provided has a step on finding the change of y. 

F(x,y)*change of x

=F(1,k)*0.5

=(2*1+k)*0.5

=1+0.5k

The question: why =(2*1+k)*0.5?