$6.$ The height of an object thrown in the air depends on the time since it has been thrown. For a particular situation the height in meters of an object after t seconds can be represented by h$($t$)=20$t $-4\mathrm{.}9.$

$($a$)$ What is the height of the object if the time is x seconds?

$($b$)$ If the time is increased by $2$ seconds, how much higher is the object?

$($c$)$ How much higher is the object per second increase?

$($d$)$ If the time is increased by h$,$ how much higher is the object?