Set the height property for elements of the mainPhoto class to 200px.

<html>
<head>
<style>
.mainPhoto {
height:200px;
width:200px;
}

img {
height:100px;
width:100px;
}
</style>
</head>
<body>
<h1>Top Destination</h1>
<img class="mainPhoto" src="https://getmimo.com/r/beach.png">
<h2>Other Destinations</h2>
<img src="https://getmimo.com/r/market.png">

</html>]]>

Make sure to pick height followed by : and then 200px, ;.