Hello, this equation came up in class:

y^4 - 400y^2 + 25600

The teacher used this method to solve it:

let y^2 = z

z^2 - 400z + 25600

z = 320 or 80

I was just wondering what the name for the method used to solve it is, so that I can google and find out more information / practice examples etc. It is the first time I had come across this type of polynominal with an exponent higher than 3.

Thanks kindly.