Question

select the gerund or gerund phrase. when you know the length of the base and the height, calculating the area of a triangle is easy.

Explanation:

A gerund is a verb form ending in -ing that functions as a noun. In the sentence "When you know the length of the base and the height, calculating the area of a triangle is easy," "calculating the area of a triangle" is a gerund phrase because "calculating" is a gerund (verb -ing acting as a noun) and it has the object "the area of a triangle" to form the phrase, which functions as the subject of the sentence.

Answer:

calculating the area of a triangle