Final answer:
To solve for st, we can subtract the equations rs - rt.
Step-by-step explanation:
The question is asking to find the value of st given that rs = 15 and rt = 18.
Given that rs = 15, we know that r + s = 15.
Given that rt = 18, we know that r + t = 18.
Subtracting the second equation from the first equation, we get:
(r + s) - (r + t) = 15 - 18
s - t = -3
Therefore, st = -3.