find the surface area of the rectangular prism.

Explanation:

Step1: Recall surface - area formula

The surface - area formula of a rectangular prism is $SA = 2(lw+lh + wh)$, where $l$ is the length, $w$ is the width, and $h$ is the height. Here, $l = 7$ km, $w = 3$ km, and $h = 5$ km.

Step2: Calculate $lw$

$lw=7\times3 = 21$ km².

Step3: Calculate $lh$

$lh=7\times5 = 35$ km².

Step4: Calculate $wh$

$wh=3\times5 = 15$ km².

Step5: Substitute into formula

$SA = 2(21 + 35+15)$. First, add the values inside the parentheses: $21+35 + 15=71$ km². Then, multiply by 2: $SA=2\times71 = 142$ km².

Answer:

$142$ km²