Althought it is not completely clear to me what your question/problem exactly is, I will give it a try I assumed that you want to solve the following equation for t:
3.7e17 = 3.88e16*(t)^(1.5)*exp(-7000./t)
Please note the use of 'e' for the scientific notations which makes its way shorter. Next to this I removed the dots because no need to solve this element wise (only scalar calculations).
The main problem with your code is that you first assign a value to to 'y' and then overwrite this value with the full equation. So introducing a different variable gives the following.
And this solves for t, (it will give warning that it is not symbolically solved but a numerical approximation is used).