Here is a question that has me puzzled

Here is a question that I am stumped on.

I put the equation into Desmos and it really is two straight lines. 

The angle between them appears to be always 90 degrees.

https://www.desmos.com/calculator/dvnoukkszg

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What is the equation of the two straight lines represented by the equation

\(ab (x^2-y^2) + (a^2-b^2)xy=0\)

Also, find the angle between them?  

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From the graph I can see that the lines are   y=mx  and y=-x/m

But how do I rearrange to get this?