What is the explicit formula for this arithmetic sequence? -11, -18, -25,-

Find three consecutive numbers such that the sum of one-fourth the first and one-fifth the second is five less than one-seventh the third

When you answer this can you explain

Is the product of two rational numbers irrational or rational? first, make a hypothesis by multiplying two rational numbers. then, use variables such as x=a/b and y=c/d and the closure property of integers to prove your hypothesis.