Calcium-51 has a half-life of 4.5 days. only 0.75 gram remains of a sample that was initially 12 grams. how old is the sample of calcium-51? 4.5 days 9.0 days 13.5 days 18 days

a charge 3q is at the origin, and a charge −2q is on the positive x axis at x=a. part a where on the x-axis would you place a third charge so it would experience no net electric force? enter an expression for the exact answer, which will involve a square-root sign. (do not enter an approximate decimal answer.)