Which term describes lines that intersects at a 90 degree angle?

The lines are Perpendicular

Perpendicular lines are lines that directly bisect each other, forming 90° angles

~Rise Above the Ordinary

you didn't post any answer choices, so its not 100% clear what your teacher is looking for

however, log(15/7) is equivalent to log(15) - log(7)

i'm using the rule log(x/y) = log(x) - log(y)

yes they are the same