Can you please help me out with this maths question?

Yes, this is a homework question. The thing is, I am the tutor, and it is my student’s question.

If y = 0.5(a^b + a^-b), show by substitution that loga(y + sqrt(y^2 – 1)) = b.

loga is log base a, and sqrt is the square root of y^2 – 1.

I’m not quite sure what they mean by “by substitution”, or exactly which direction I should take this in. Thanks in advance!