Simple...
you have: -7=c-6
You want to isolate the variable c
-7=c-6
-7=c-6
+6 +6
-1=c
Thus, your answer.
Assuming that the two investments are X & Y
X + Y = 6300
X = 6300 - Y (1)
9/100X + 4/100 Y = 372 (2)
replacing X from (1) into (2)
9/100(6300-Y) + 4/100 Y = 372
567 - 9/100Y +4/100Y = 372
(-9+4)/100Y = 372 - 567
5/100Y = -195
Y = 100*195/5 = 3900
From (1) we can get X
X = 6300 - 3900 = 2400
I hope this is helpful
