Question

the sum of two numbers is 30 and their difference is 6.Find the number.Let x be the large number making x-? the smaller number

Answer 1

So you know that
x+(x-?)=30
x-(x-?) = 6

You now have 2 equations

Answer 2

For easier reading we'll pretend x-? is the same thing as y

x+y=30
x-y=6->x=6+y

6+y+y=30
6+2y =30
2y =24
y = x-? = 12