Pell Equations: $a^2+4=5b^2$ This is a challenge problem in the Pell Equations chapter of my number theory book, but I'm not seeing the connection to Pell Equations. The Pell Equation with the coefficient $5$ is $5b^2...--prophetes.ai

Pell Equations: $a^2+4=5b^2$ This is a challenge problem in the Pell Equations chapter of my number theory book, but I'm not seeing the connection to Pell Equations. The Pell Equation with the coefficient $5$ is $5b^2+1=a^2$, but it doesn't look like the one I have. Thanks if you can help me.

if a,b are natural numbers :$$a-b