\(\text{a},=7.12+16:8=84+2=86\\ b,=72:\sqrt{4.36.9}-15=72:\sqrt{1296}-15=72:36-15=2-15=-13\)
a) \(\sqrt{49}.\sqrt{144}+\sqrt{256}:\sqrt{64}=\sqrt{7^2}.\sqrt{12^2}+\sqrt{16^2}:\sqrt{8^2}=7.12+16:8=84+2=86\)
b) \(72:\sqrt{2^2.36.3^2}-\sqrt{225}=72:\sqrt{2^2.6^2.3^2}-15=72:2:6:3-15=2-15=-13\)