Answer:
The area of the triangle is of 21 units of area.
Step-by-step explanation:
The area of a triangle with three vertices 
is given by the determinant of the following matrix:
In this question:
Vertices (3,0) (9,0) (7,6). So
The area of the triangle is of 21 units of area.
X is 0,in most situations,but could you provide a picture next time?
Simply distribute the -3 across the b and -7.
-3(b - 7) = (-3 * b) + (-3 * -7) = -3b + 21
Hope this was helpful, if it wasn't just drop me a DM explaining what didn't make sense.
The answer would be group A, hope this helps.
