35.84 is the answer
When adding using the significant figure rules, first you have to identify which term is the limiting term. This can be determined by identifying with number has the least amount of numbers after the decimal point.
- 31.580 has three numbers after the decimal point (580)
- 4.26 has two numbers after the decimal point (26)
This makes 4.26 the limiting term. This means your final answer should be rounded up so there are also 2 numbers after the decimal point.
31.580 + 4.26= 35.840
Round up so there are only two numbers after the decimal point, which leaves you with the answer 35.84