Answer:
10√7
Explanation:
To combine the radicals, you must have the same number under both radicands. Luckily, you can simplify one of the radicands. This is done by separating the radicand (√28) into its common factors (√4 x √7) and taking the square root of one of the factors (√4 = 2).
4√7 + 3√28 <---- Original expression
4√7 + 3(√4 x √7) <---- Expand √28
4√7 + (3 x 2)√7 <----- Take √4
4√7 + 6√7 <---- Multiply coefficients
10√7 <---- Add both terms