You check to see what you can extract from those radicals.

You expression looks like this

##sqrt(18) – sqrt(50)##

Write the prime factors of these two numbers to see if you can find write tham as products of a perfect square and another number

##18 = 2 * 9 = 2 * 3 * 3 = 2 * 3^2##

##50 = 2 * 25 = 2 * 5 * 5 = 2 * 5^2##

You can thus write, with the help of the product property of radicals, an equivalent expression that looks like this

##sqrt(2 * 3^2) – sqrt(2 * 5^2) = sqrt(2) * sqrt(3^2) – sqrt(2) * sqrt(5^2)##

Finally, you get

##3 * sqrt(2) – 5 * sqrt(2) = color(green)(-2sqrt(2))##