Yes it's a function, because for each input of x value, we have exactly an output of y-value
-2(7-y)+4=-4
Distributive property for -2(7-y)
-14+2y+4=-4
Addition property
-14+14+2y+4=-4+14
2y+4=10
Subtraction property
2y+4-4=10-4
2y=6
Division property
2y/2=6/2
Y=3