Looked for this all over the web. If you wanted to say "evaluated at "b" upper bound, and "a" lower bound" (assuming the integral had already been solved, except for plugging in limits) then how could you express as latex?

$\displaystyle \int_{a}^{b} 3 dx$

$\displaystyle = 3x $ evaluated at "b" upper bound, and "a" lower bound.