Question

y=-x+4if y =7

Answer 1

You are looking for a value of x?

Answer 2

so you have to plug in 7 to the equation
7=-x+4
so then you want to get 4 out of the equation, you have to to do the opposite of +4 and subtract by 4
7= -x+4
-4 -4
3= -x
now u divide by negative one to get the -x to x
3/ -1 x/ -1
-3 = x
hope this helped! :D