You use ##\LaTeX##, for which see the
guidelines: (point 6)
(##\TeX## commercial:)
It's so easy ! just put ## or $$ around the expression to get ##LaTeX## .
So $$(aB/2)t^2 + vB(initial)*t - vA(initial)*t + xBi = 0$$ becomes $$(aB/2)t^2 + vB(initial)*t - vA(initial)*t + xBi = 0$$
Then: superscripts come with ^ and subscripts with _ and grouping goes with curly brackets {...}
You learn from other posts with a right-click on the formula and picking Show Math as ... ##\TeX## commands.
For your question: \frac {a+2} {b} gives ## \frac {a+2} {b} ## and {1-2-3\over 4-5-6} gives ##{1-2-3\over 4-5-6} ##
[edit] by the way:

! And don't hesitate to post whatever. We are used to imperfection (we only strive for perfecetion

)