Unicode isn't meant to replace all typesetting like LaTeX. For example, I can't make proper horizontal fractions (as opposed to slashed like ⅝ or ⁹⁄₁₆) that are normal in my part of the world because that would be too much scope creep. Even the TeX logo is not really doable (Markdown with ASCII: ^T^~E~^X^, Unicode ᵀᴇˣ).

An imperfect solution is adding ̅ U+0305 COMBINING OVERLINE above everything. For example, it does not sit at consistent height (√4̅a̅c̅) and Windows renders it incorrectly (centered to the right edge of the character, not its center).

This is how I'd render the numerator using Unicode only:
𝑏² ± √4̅𝑎̅𝑐̅

A correct Markdown interpreter can improve the look ogf the superscript:
𝑏^2^ ± √4̅𝑎̅𝑐̅