HTML is rather limited when it comes to mathematical expressions. In principle, HTML specifications suggest that you use sup element for superscripts, so the sample expression would be
(<i>a</i> + <i>b</i> + √<i>c</i>)<sup>2<i>x</i> + <i>b</i></sup>
However, the implementations of sup in browsers are inconsistent and generally poor (causing e.g. uneven line spacing), so it is pragmatically better to use the span element with a class instead:
(<i>a</i> + <i>b</i> + √<i>c</i>)<span class=sup>2<i>x</i> + <i>b</i></span>
with CSS code like
.sup {
position: relative;
bottom: 1ex;
font-size: 80%;
}
Some reasons are explained on my page Math in HTML (and CSS). Also consider JavaScript-based libraries for pages containing complicated math expressions:
- KaTeX
- MathJax
- jqMath
The sample expression is a borderline case; it would look mathematically more correct if the square root were represented using a square root symbol with vinculum and not just √c, and trying to construct a vinculum using HTML and CSS gets rather dirty, but otherwise it can be reasonably handled with HTML and CSS.