you have to use pythagorean theorem which means a^2+b^2=c^2 because no angle is given
c is going to be the longest side or opp to 90 degree.
so c=13 and b=12
a^2+ (12^2)= 13^2
a^2 + 144 = 169
a^2=25
a=5
solve for a, can you do that?
a=5
hope that helps