Which expression represents the largest number? a. 6.1x10^-7 b.4.1x10^-7 c.3.1x10^-8 d.5.1x10^-8

Since you're looking for the largest write out the numbers in standard form. 

A=0.00000061 or 6.1 x 10^-7
B=0.00000041 or 4.1 x 10^-7
C=0.000000031 or 3.1 x 10^-8
D=0.000000051 or 5.1 x 10^-8

So as you can see C and D are the lowest numbers since the exponent is -8. A and B both have the same exponent -7, although 6.1 is a bigger number therefore it is the largest.