You can calculate BMI using this formula:
a) To conclude you need to compare the BMI of this year old boy with the normal BMI for this age (you should have a table with normal BMI or you can search it on the internet) . If it is higher it can be overweight or obese, if it is lower compared to the normal BMI it is underweight. From what I saw when BMI is above 24.9 it is considered overweight.
b) In this exercise you have to solve the equation, you consider that weight is an X and take into account units