Suppose

y $=$

$2$x $+1$

$,$

where x and y are functions of t$.$

$($a$)$ If

dx$/$dt $=15,$

find

dy$/$dt

when

x $=4.$

dydt

$=$

$($b$)$ If

dy$/$dt $=3,$

find

dx$/$dt

when

x $=40.$

dxdt

$=$