What is the value of the function v=5/2t-3/4 when t=3?

I can interpret you're question either as v=5 / (2t) - 3/4, or v = (5/2) t-3/4.

1. v=5 / (2t) - 3/4, v=5/6-3/4, v=10/12-9/12, so v=1/12.

2. v = (5/2) t-3/4, v=15/2-3/4, v=30/4-3/4, so v=27/4 or 6 3/4