Supplement of (270-a) degree
To find the supplement of an angle, we subtract the angle from 180 degrees. The supplement of an angle and the angle itself add up to 180 degrees. So, the supplement of angle (270-a) would be (180 - (270-a)) degrees.

Calculation
Subtracting (270-a) from 180:
Supplement = 180 - (270 - a)
Supplement = 180 - 270 + a
Supplement = -90 + a
Supplement = a - 90
Therefore, the supplement of (270-a) degrees is (a - 90) degrees.

Explanation
When we have an angle (270-a), the supplement of that angle is found by subtracting the angle from 180 degrees. This is because the supplement of an angle is the amount of degrees needed to make the angle a straight line (180 degrees). In this case, by subtracting (270-a) from 180, we find the supplement to be (a - 90) degrees.
In summary, the supplement of angle (270-a) degrees is (a - 90) degrees.