Question

if 3a+1=x and 3a=y,express y in terms of x

Answer 1

3a = y -- sub 3a in for y in the other equation

3a + 1 = x
y + 1 = x
y = x - 1 or x = y + 1

Answer 2

in terms of x, I guess it would be : x = y + 1

Answer 3

in terms of x it would be y=x-1.

The question asked you to express y. That normally means "y= (some expression)"