Question

Find the lateral area of a regular square pyramid whose slant height is 20 and whose base has an edge length of 7

Answer 1

Lateral area does not include the base, so we want the total area of just the four triangles that make up the other faces.
These triangles are isosceles and congruent to one another, since this square pyramid is regular.
Draw one of these isosceles triangles and label it with the measurements in your question. You should soon see how to work out the area of that triangle.
Then simply multiply your result by 4.

Answer 2

the slant height of the pyramid is the altitude of each triangular face.

Now you have four triangles, and you know their base and height.

I assume you can take it from here.