Set both left and right margins to 90px.

<html>
<head>
<style>

.general {
margin-left: 90px;
margin-right: 90px;
}

.lieutenantGeneral {
margin-left: 30px;
margin-right: 30px;
}

p{
text-align:center;
background-color: lightblue;
border: 1px solid black;
}

.majorGeneral {

</style>
</head>
<body>

<h2>Military General Ranking</h2>

<p class="general">General</p>

<p class="lieutenantGeneral">Lieutenant General</p>
<p class="majorGeneral">Major General</p>

</body>
</html>
]]>

Make sure to pick 90px, followed by 90px.