`(-5-sqrt3)^2`

Squaring an expression is the same as multiplying it 2 times.

`(-5-sqrt3) (-5-sqrt3)`

Now use the FOIL method.

`(-5*-5)(-5*-sqrt3)(-sqrt3*-5)(-sqrt3*-sqrt3)`

Simplify by multiplying and combining like terms.

`(25+10sqrt3+sqrt3^2)`

Raising a square root to the square power results in the expression inside the root.

`25+10sqrt3+3`

Add 3 to 25 to get 28.

`28+10sqrt3`

** `(-5-sqrt3)^2= 28+10sqrt3`**