Which rule has initial value (90) and decay rate (6)?

Correct answer: \(y=90-6x\)

A linear rule has the form \(y=a+mx\), where \(a\) is the initial value and \(m\) is the rate of change. A decay rate of 6 means that the rate is \(-6\). Therefore, the rule with initial value 90 is \(y=90-6x\). In \(y=90+6x\), the value increases rather than decays. Exam tip: check the constant term for the initial value and a negative coefficient of \(x\) for decay.