Answer
$-a^{2}-b^{2}$
Work Step by Step
$(2a^{2}-ab+6b^{2})+(-3a^{2}+ab-7b^{2})$
Remove both parentheses:
$2a^{2}-ab+6b^{2}-3a^{2}+ab-7b^{2}=...$
Combine like terms:
$...=(2-3)a^{2}+(1-1)ab+(6-7)b^{2}=...$
Simplify:
$...=-a^{2}-b^{2}$
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