In this problem we must first convert to scientific notation;

0.000000074 = 7.4 x 10-8

0.000000555666777888 = 5.55666777888 x 10^-7

The terms now have different powers, and so can not be subtracted directly. Instead one term must be converted

7.4 x 10-8 - 5.55666777888 x 10^-7

= 0.74 x 10-7 - 5.55666777888 x 10^-7

= (0.74 - 5.55666777888) x 10^-7

= - 4.8166777888 x 10^-7