Simplest form of the expression 3sqrt750+3sqrt2058-3sqrt48 can someone explain

First, you have to decompose the numbers under the square root:
750 = 2·3·5²
2058 = 2·3·7³
48 = 2⁴·3

Then, enlight all the exponents multiple of 2 (which is the index of the root):
3·√(2·3·5²) + 3·√(2·3·7²·7) + 3·√(2⁴·3)

At this point, you can take outside the square root everything that has an even number, and doing so you have to halve the exponent; you'll get:
3·5·√(2·3) + 3·7·√(2·3·7) + 3·2²·√3

You can now multiply:
15√6 + 21√42 + 12√3
which is the simplest form of your expression.