I'm going to assume you just mean how do you write the ratio, in which case the answers vary
18:6
18 to 6
or
18/6
There is no square root of 6 all you have to do is set up a square root box and put the 6 in it then you split it into factors next you put them in another square root box finally multiply, then you get your simplest form.
Step-by-step explanation:
Let the required number be x.

A = (H * B) / 2.......2A = H * B
A = 15
H = B + 1
2(15) = H * B
30 = B(B + 1)
30 = B^2 + B
B^2 + B - 30 = 0
(B + 6)(B - 5) = 0
B + 6 = 0
B = -6.....not this one, it cant be negative
B - 5 = 0
B = 5 <==== base is 5 cm
H = B + 1
H = 5 + 1
H = 6 <=== height is 6 cm