-1 - (-7) = 6
5 - (-1) = 6
11 - 5 = 6
The common difference is 6.
If you add 6 to a term, you get the next term.

DEF= 6 ft^2
A= 1/2*b*h
Plug in what you know.
6= 1/2*4*h
Multiply like terms.
6=2*h
Divide 2 on both sides.
3=h
If the base of ABC= 12
12/4=3
3*3=9
A= 1/2*b*h
A= 1/2*12*9
A= 1/2*84
A= 42
The area of ABC is 42 ft^2
I hope this helps!
~kaikers