To find "s", you can use the Pythagorean theorem(which you can only use for right triangles):
a² + b² = c² (c is the hypotenuse/the longest side of the triangle)
t = 18 (hypotenuse)
r = 12
s = ?
Plug in the numbers into the equation
a² + b² = c²
(12)² + b² = (18)²
144 + b² = 324 Subtract 144 on both sides
b² = 180 Square root both sides
b = √180 or 13.4