Optimisation help (differentiation)?

[Emma]

This really confuses me, it's the hardest part of maths in my opinion.
This is the question:
A cuboid of volume 1125cm3 has length equal to three times the breadth.
A) show that the surface area of the cuboid may be expressed as: 6x^2 + 3000/x
B) find the dimensions of the cuboid that give the minimum surface area.

Could someone explain this so I understand it? Thank you!

[beauty]

yes