To isolate b, add a and subtract c to the entire equation:
D - a + c = -b
Muliply b by -1 to eliminate the negative:
-D + a - c = b
Equation parallel to y = -x is y=-x+k
so 4.5 = -7 + k so k - 11.5
Equation is y = -x + 11.5
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It doesnt make any sense because the diagram is wrong so I'm guessing it would be either 8 or 6. Im sorry that problem is wrong :(