9*4+6\2=45 where do you put the perentizes

Could somebody me with this radical simplification?

X+y=-4 x-y=2 use elimination with adding and subtraction

How many 3-digit numbers can you write using only digits 1 and 7? (of course, digits can repeat.) how many 3-digit numbers can you write using only digits 1 and 7? (of course, digits can repeat.)