Need with this problem explain pls

All sides of an equilateral triangle are equal, so all three sides are 9 cm.
To find h, use the pythagorean theorem. This requires use of only one right triangle, where we use b as our h.

a^2+b^2=c^2
(4.5)^2+b^2=(9)^2
(20.25)+b^2=(81)
b^2=(81/20.25) or 4
square root both sides
b=2