the way m comfortable solving this is using pythagorean therem.
the imaginery hight line you draw in the middle of the triangle should split the 6cm base in half to 3cm and the outside of the tryangle is 6cm.
pythagorian theorem formula states:
c^2 =a^2 +b^2
c=6cm
b = 3cm
a=?
6^2 =3^2+a^2
36=9+a^2
36-9=a^2
a^2 =27
square root 27 to find a
=5