the oppsosite of a number is defined as the number that when added to the original gives zero. in this context, it's just the negative of the number.

let the number be x

so 5 times the number is 5x and that is greater than -2x (twice the opposite of x) by 35, so if we add 35 to -2x we get 5x

so 5x = 35 - 2x

=> 7x = 35 ...........added 2x to both sides, 5x + 2x = 7x

=> x = 35/7 ..........divided both sides by 7

=> x = 5