To find a, we will use the Pythagoras theorem,
adjacent² + opposite² = hypotenuse²
From the diagram, opposite = 5 adjacent = a hypotenuse =13
substitute the values into the formula and evaluate
a² + 5² = 13²
a² + 25 =169
subtract 25 from both-side of the equation
a² = 169 - 25
a² =144
Take the square root of both-side
a = 12