You should notice that it is also

-4 + log 6.70 = -4 + 0.826 = -3.174

Note: The number of significant figures in a logarithm is only the number of places after the decimal point since the integer part only gives the power of 10. All the numbers in the question (except (b)) are given to 3 figures and so the answers are gi ven to 3 figures.