Can someone please help with this I am confused about how to know which side is a and c

b=4

Step-by-step explanation:

The side opposite the right angle is the hypotenuse.  The sides touching the right angle are the legs

Since this is right triangle we can use the Pythagorean theorem

a^2+b^2 = c^2  where a and b are the legs and c is the hypotenuse

3^2+b^2 = 5^2

9+b^2 = 25

Subtract 9 from each side

9-9+b^2 = 25-9

b^2 =16

Take the square root of each side

sqrt(b^2) = sqrt(16)

b=4