1. The problem asks to find the value of the expression $s + t$ given $s = 3$ and $t = 5$. 2. The formula to use is simple addition: $$s + t$$ 3. Substitute the given values into the expression: $$3 + 5$$ 4. Add the numbers: $$3 + 5 = 8$$ 5. Therefore, the value of the expression $s + t$ when $s = 3$ and $t = 5$ is 8.