(14.007 g/mol + 3.02 g/mol)(8.60791 mol)

Significant Figure Rules: 
For Addition/Subtraction you round to the last place value you have in common. In the case of:
14.007 g/mol + 3.02 g/mol 

It is the Hundreth spot.
So... your answer to that part of the problem would come out as: 
17.027 g/mol, but you need to round it to the nearest hundreth. 
So, 17.03 g/mol is the correct amount. 

(17.03 g/mol)(8.60791 mol)

For multiplication you follow the significant figure rules. In this case 17.03 g/mol has the least amount of significant figures (4) so we'll round to that many. 

146.6 g is the answer!

Note that the answer, 146.6g, has lost the moles, because we canceled them out. (It came out to be 146.6 g*mol/mol, and when something is it over itself it cancels out!) 

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