There was a sample of 350 milligrams of a radioactive substance to start a study. Since then, the sample has decayed by 6.5 % each year. Let t be the number of years since the start of the study. Let y be the mass of the sample in milligrams. Write an exponential function showing the relationship between y and t.

Step 1 ：The problem is asking for an exponential function that models the decay of a radioactive substance. The general form of an exponential decay function is \(y = a \times (1 - r)^t\), where \(a\) is the initial amount, \(r\) is the rate of decay, and \(t\) is time.

Step 2 ：In this case, \(a\) is 350 milligrams, \(r\) is 6.5%, and \(t\) is the number of years since the start of the study.

Step 3 ：We need to convert the decay rate from a percentage to a decimal, so \(r = \frac{6.5}{100} = 0.065\).

Step 4 ：Therefore, the function is \(y = 350 \times (1 - 0.065)^t\).

Step 5 ：The exponential function showing the relationship between \(y\) and \(t\) is \(\boxed{y = 350 \times (1 - 0.065)^t}\).