$a)72\cdot {10}^6+8\cdot {10}^5=720\cdot {10}^5+8\cdot {10}^5=\left(720+8\right)\cdot {10}^5=728\cdot {10}^5=7,28\cdot {10}^7$  

$b)13\cdot {10}^{10}+0,23\cdot {10}^{12}=13\cdot {10}^{10}+23\cdot {10}^{10}=\left(13+23\right)\cdot {10}^{10}=36\cdot {10}^{10}=3,6\cdot {10}^{11}$  

$c)8\cdot {10}^7-3,5\cdot {10}^5=800\cdot {10}^5-3,5\cdot {10}^5=\left(800-3,5\right)\cdot {10}^5=796,5\cdot {10}^5=7,965\cdot {10}^7$  

$d)2\cdot {10}^{-7}-8\cdot {10}^{-8}=20\cdot {10}^{-8}-8\cdot {10}^{-8}=\left(20-8\right)\cdot {10}^{-8}=12\cdot {10}^{-8}=1,2\cdot {10}^{-7}$