$\sqrt{x}=t+7$

$\Rightarrow x=(t+7)^2$

$=t^2+49+14 t \text { (squaring) }$

$\frac{d x}{d t}=2 t+14$

$v=2 t+14 \Rightarrow v \propto t$

Acceleration :

$a=\frac{d v}{d t}$

$a=2 \,ms ^{-2} \rightarrow \text { constant }$