we have
(4.7 x 10^7) + (3.4 x 10^6)
To adds the number the exponent must be the same
so
we know that
3.4 x 10^6=0.34 x 10^7
substitute
(4.7 x 10^7) + (0.34 x 10^7)=(4.7+0.34) x 10^7
=5.04 x 10^7
we have
(9x10^5)-(8x10^4)
we know that
8x10^4=0.8x10^5
substitute
(9-0.8)x10^5
=8.2x10^5
we have
(2.8x10^-5)+(5.4x10^-6)
we have that
2.8x10^-5=28x10^-6
substitute
28x10^-6+5.4x10^-6
=33.4x10^-6
=3.34x10^-5