Draw a right triangle ABC with angle A=45° ,angle B=90° and angle C=45° ( rough diagram is enough)
According to pythagoras theorem we have,
AC^2=AB^2 + BC^
Here we are given ,
AB=BC=y
AC^2=Y^2 + Y^2
AC^2=2y^2
Taking square root on both sides we get
AC=(sqrt 2)y (which is the length of the hypotenuse)