if a number is decreased by 5, and then the result is multiplied by 2, the result is 26

If a number is decreased by 5, and then the result is multiplied by 2, the result is 26

The phrase a number means an arbitrary variable, let's call it x:
x

Decreased by means we subtract 5 from x:
x - 5

Multiply the result by 2:
2(x - 5)

The result is 26 means we set 2(x - 5) equal to 26:
2(x - 5) = 26