Since it is a right triangle we can apply trigonometric formulas:
Sin a = opposite side / hypotenuse
Where:
a= angle = 63°
opposite side = 16
Hypotenuse = x (the longest side)
Replacing:
Sin 63 = 16 / x
Solve for x
x = 16 / sin 63
x = 17.95 = 18 (rounded)
x = 18