The sum of x and 10 is less than -6

Try breaking these into sections.

Twice: 2 times

The sum of: add

A number: choose a variable, like x

---Thus "the sum of a number and 4" becomes "add x and 4" which, mathematically, is "x+4"

Continuing to put it all together, "Twice the sum of a number and four" becomes "2 times (x+4)" which, mathematically, is "2(x+4)"

Is: equals

"Twice the sum of a number and four is" becomes "2(x+4)="

23 less than: subtract 23. This one tends to trick people; "23 less than" will become "__ - 23", NOT "23 - __"

three times the number: 3 times x

---"23 less than three times the number" becomes "subtract 23 from 3 times x" which, mathematically, is "3x-23"

So the final phrase: 2(x+4)=3x-23"

The correct one is c.