With the data we have, we have to
A + B = - 10
A - B = 4
A . B . C = 40
We want the value of C. This is
Isolate A
A = - 10 - B
Replaces in A - B = 40, have to
- 10 - B - B = 4
- 2 B = 14
B = - 7
Replaces B
A = - 10 - ( - 7 )
A = - 10 + 7
A = - 3
like this
- 7 . ( - 3 ) . C = 40
21 . C = 40
C = 40 / 21
Hope that is helps, good studies.