\(1.\\8+a=3.9\ \ \ |subtract\ 8\ from\ both\ sides\\\\\boxed{a=3.9-8}\to\boxed{a=-4.1}\\\\2.\\3.9+a=8\ \ \ |subtract\ 3.9\\\\\boxed{a=8-3.9}\to\boxed{a=4.1}\\\\3.\\3.9+a=-8\ \ \ |subtract\ 3.9\ from\ both\ sides\\\\\boxed{a=-8-3.9}\to\boxed{a=-11.9}\\\\4.\\4.8+a=-3.9\ \ \ |subtract\ 4.8\ from\ both\ sides\\\\\boxed{a=-3.9-4.8}\to\boxed{a=-8.7}\)
