已知 $3v=\sqrt{25-x^2}$，

1. Substitute $x=-3$ into the given equation and evaluate.

Given $3v=\sqrt{25-x^2}$,

when $x=-3$,
$3v=\sqrt{25-(-3)^2}=\sqrt{25-9}=\sqrt{16}=4.$

2. Solve for $v$ by dividing both sides by 3.

$3v=4$
$v=\frac{4}{3}.$

1. Square both sides to remove the square root.

$3v=\sqrt{25-x^2}$

Squaring both sides,
$9v^2=25-x^2.$

2. Rearrange the equation to make $x^2$ the subject.

$9v^2=25-x^2$
$x^2=25-9v^2.$

3. Take the square root to make $x$ the subject.

$x=\pm\sqrt{25-9v^2}.$