is this script right?

2+2=4
if (2 + 2 == 4) then
print("2+2=4. This is correct")
else
print("2+2 is not 4 it is 9. Check your math")
end

Yes, except remember that the first line is not valid Lua.

If it's simply a remark about the code (like it is) then it should be in a comment, specified with two hyphens:

-- 2 + 2 = 4

As a stylistic note, the parenthesis around 2 + 2 == 4 are not required and would usually be omitted.

In addition, it is best practice to tab code to make the "blocks" (the things which are part of if, while, for, function, etc) more obvious:

-- 2+2=4
if 2 + 2 == 4 then
	print("2+2=4. This is correct")
else
	print("2+2 is not 4 it is 9. Check your math")
end