The distance between points $ A $ and $ B $ is :
$$ d(A, B) = 5 $$To find distance between points $ A(x_1,y_1)$ and $ B(x_2,y_2)$, we use formula:
$$ \color{blue}{ d(A,B) = \sqrt{(x_2 - x_1)^2+(y_2 - y_1)^2} } $$In this example we have:
$$ \begin{aligned} & A \left(4,~0\right) \implies x_1 = 4 ~~\text{and}~~ y_1 = 0 \\[1 em] & B \left(0,~3\right) \implies x_2 = 0 ~~\text{and}~~ y_2 = 3 \end{aligned} $$Substituting $ x_1 $, $ x_2 $, $ y_1 $ and $ y_2 $ into the formula above yields:
$$ \begin{aligned} d(A,B) & = \sqrt{\left( 0 - 4 \right)^2 + \left( 3 - 0 \right)^2} \\[1 em] d(A,B) & = \sqrt{ (-4)^2 + 3^2 } \\[1 em] d(A,B) & = \sqrt{ 16 + 9 } \\[1 em] d(A,B) & = \sqrt{ 25 } \\[1 em] d(A,B) & = 5 \end{aligned} $$