Subtract. Then write the difference in lowest terms. \[ \frac{21}{25}-\frac{6}{25}= \]

Step 1 ：The problem is asking to subtract two fractions that have the same denominator. In such a case, we simply subtract the numerators and keep the denominator the same.

Step 2 ：Subtract the numerators: \(21 - 6 = 15\).

Step 3 ：So, \(\frac{21}{25}-\frac{6}{25}=\frac{15}{25}\).

Step 4 ：After that, we simplify the fraction if possible.

Step 5 ：Both the numerator and the denominator can be divided by 5.

Step 6 ：So, \(\frac{15}{25} = \frac{3}{5}\).

Step 7 ：Final Answer: The difference in lowest terms is \(\boxed{\frac{3}{5}}\).