Easy way
-21÷3=-7 which is the exact center so
-6, -7, -8
Equation
x + (x+1) + (x+2) = -21
3x + 3 = -21
3x=-24
x=-8
x+1=-7
x+2=-6
2. Consecutive Odd Integers
X, X+2, X+4
X + (X+2) + (X+4) = 144
3X+6=144
3X=144-6=138
X = 138/3 = 46 .....There are no 3 odd Integers that can equal an even integer
3. 3 consecutive even number x, x+2, x+4
X + (X+2) + (X+4) = 48
3X+6=48
3X=48-6=42
X=42/3=14
14, 16, 18
A rational number is a number that can be expressed as the relationship between two integers. Fractions and decimals will either repeat or end. An example is 5. Maybe 7. 53573 is an example as well. So is 5/1, because 5 and 1 are both integers. so is 7.5, because 7 and 5 are both integers, and it ends (the numbers after the decimal doesn't keep going). Your answer is 7.8, because it ends.
Pi: Irrational
√(2/3): Irrational
0.57575757 is rational
and -1/2 is rational.
I hope this helps.