<p class="left_margin"> 5)
<img src="https://sait.ru/formula/svg/8b/8b847cfae498e96e3fa08a56cb42b838.svg" class="tex" style="vertical-align:0pt" />
    blablabla
</p>

var html = @"
<p class='left_margin'> 5)
<img src='https://sait.ru/formula/svg/8b/8b847cfae498e96e3fa08a56cb42b838.svg' class='tex' style='vertical-align:0pt' />
    blablabla
</p>
";

var doc = new HtmlDocument();
doc.LoadHtml(html);

var p = doc.DocumentNode.SelectSingleNode("//p");

var textNodes = p.SelectNodes("text()")
    .Select(n => n.InnerText.Trim())
    .ToList();

var src = p.SelectSingleNode("img").Attributes["src"].Value;

Console.WriteLine(textNodes[0]); // 5)
Console.WriteLine(textNodes[1]); // blablabla
Console.WriteLine(src); // https://sait.ru/formula/svg/8b/8b847cfae498e96e3fa08a56cb42b838.svg

var textNodes = p.ChildNodes
    .Where(n => n.NodeType == HtmlNodeType.Text)
    .Select(n => n.InnerText.Trim())
    .ToList();

var nodes = p.ChildNodes
    .Select(n => n.NodeType == HtmlNodeType.Text ? n.InnerText.Trim() : n.Attributes["src"].Value);

string result = string.Join(" ", nodes);