When 40% of a number is added to the number the result is 210

So it gives you the "= 210" part of it so you just need to figure out how to write "40% of a number is added to the number"

the questions already tells you that "the number" is "x"

so that means "40% of x is added to x"
instead of writing "add" you can replace it with a plus sign "+"
(40% of x) + x
another way to write 40% of x is 0.40x
so your equation is .4x + x = 210