The hypotenuse is the longest side of a triangle, and it connects the leg and the base usually. In this triangle, 18 is the hypotenuse -- so since we are solving for the unknown side, the equation is:
a^2 + 5.7^2 = 18^2 -- now solve
a^2 = 18^2 - 5.7^2
a^2 = 291.51
a = sqrt(291.51)
a = 17.07 -- because height is the square root of the difference of the squares of 18 and 5.7
