Help please!! i need to find the value of x

After two years, how much has been paid into interest?

Initially 100 milligrams of a radioactive substance was present. after 6 hours the mass had decreased by 4%. if the rate of decay is proportional to the amount of the substance present at time t, find the amount remaining after 24 hours. (round your answer to one decimal place.)

For what values of the variable x is the expression square root of 7x+14+ square root of 7-x a real number?