The average births per woman $in$ a certain developing country can $be$ modeled according $to$ the equation

$f(x)=0\mathrm{.}008x^2-0\mathrm{.}45x+7\mathrm{.}4$ where $xis$ the number $of$ years since $1956.$

Step $2of2$: What was the rate $of$ change $in$ the country's average births per woman $in1978?$