Answer:
(6y +0.4x)²
Explanation:
Find the terms that are perfect squares and take their square roots. Combine those terms using the sign of the "xy" term.
The square terms are 36y² and 0.16x². Respectively, their roots are 6y and 0.4x. The sign of the xy term is positive, so the square you're looking for can be written ...
(6y +0.4x)² = 36y² +0.48xy +0.16x²