$1,521\cdot {10}^{8}km-1,471\cdot {10}^{8}km=\left(1,521-1,471\right)\cdot {10}^{8}km=0,05\cdot {10}^{8}km=5\cdot {10}^{-2}\cdot {10}^{8}km=5\cdot {10}^{6}km=5000000km$