Most straightforward way is to use the property a² - b² = (a - b)(a + b)
First, factorise 720. Rewrite it as a multiple of 5 so we can group it together with the other term.
5 × 88² - 720 = 5 × 88² - 5 × 144 = 5 × (88² - 144)
Next, realise that 144 is a square number. 144 = 12 × 12 = 12² (you should know your multiplication table up to 12 pretty well)
= 5 × (88² - 12²)
Here is where you use the property mentioned above.
= 5 × (88 - 12)(88 + 12)
= 5 × (76)(100)
Now you have a very nice result so just multiply 5 by 76 first
= 380 × 100
= 38000 (×100 is simply adding two 0's behind)