Step 1: Make all the exponents the same To add or subtract numbers in scientific notation we must make all the exponents the same:
$$
\begin{array}{l}
1,99 \times 10^{-26}=0,199 \times 10^{-25} \text { and } \\
1,67 \times 10^{-27}=0,0167 \times 10^{-25}
\end{array}
$$
Step 2 : Carry out the addition and subtraction Now that the exponents are the same we can simply add or subtract the $N$ part of each number:
$$
0,199+0,0167-2,79=-2,5743
$$
Step 3: Write the final answer To get the final answer we put the common exponent back:
$$-2,5743 \times 10^{-25}$$