Respuesta :

Ok 
First factor out the first 2 terms as follows:-

y  = 1/2(x^2 - 8x) + 21

Now complete the square on x^2 - 8x:-

y = 1/2[ (x - 4)^2 - 16] + 21

y = 1/2(x - 4)^2 - 8 + 21

y = 1/2(x - 4)^2 + 13   Answer