Question 310610
Use the Pythagorean Theorem.

a^2 + b^2 = c^2

a^2 + (2)^2 = (√13)^2

We are searching for side a.

a^2 + 4 = 13

a^2 = 13 - 4

a^2 = 9

Take the square root of both sides.

sqrt{a^2} = sqrt{9}

a = 3

Done!