Squaring a 2-digit number beginning with 5

Step 1:Take a 2-digit number beginning with 5.
Step 2: Square the first digit.
Step 3: Add this number to the second number to find the first part of the answer.
Step 4:Square the second digit: this is the last part of the answer.

Example:
If the number is 58, multiply 5 × 5 = 25 (square the first digit).
25 + 8 = 33 (25 plus second digit).
The first part of the answer is 33   3 3 _ _
8 × 8 = 64 (square second digit).
The last part of the answer is 64    _ _ 6 4
So 58 × 58 = 3364.