Answer quick helllpp solve. 7 ( n + 4 ) = 21 a. n = 10 b. n = 7 c. n = -1 d. n = 1

When you answer this can you explain

David is spreading mulch on a triangular area of his flower bed. the coordinates of the vertices of the area are (1, 3), (9, 3), and (4, 6). what is the area of the triangle if each square has an area of 3 square feet?

Ineed to know the answer to this algebra question.