You might know that sec²(x) - tan²(x) = 1. If we solve for sec²(x), then we obtain:
sec²(x) = 1 + tan²(x)
Then,
csc²(x)(1 + tan²(x))
==> csc²(x) + csc²(x)tan²(x)
==> csc²(x) + 1/sin²(x) * sin²(x)/cos²(x)
==> csc²(x) + 1/cos²(x)
==> csc²(x) + sec²(x)
==> RHS
I hope this helps!