A = B + (.40) C, A = 10, and B = - 6

Are you trying to solve for c?

If you are, you can rewrite a=b + (.40) c as 10=-6+0.4c because a is 10 and b is - 6.

10=-6+0.4c Add 6 to both sides to get rid of the - 6.

+6 + 6

16=0.4c Divide both sides by 0.4 to isolate c.

40=c