Answer
60.0
Work Step by Step
Using the Pythagorean Theorem, we can plug in our variable and then solve:
$${a}^2+{b}^2 = {c}^2$$
$${45.0}^2+{b}^2 = {75.0}^2$$
Rearranging gives,
$${b} = \sqrt{{75.0}^2-{45.0}^2}$$
The answer, to 3 significant digits is
$$b = 60.0$$
