What is x expressed in terms of y
y=45+6x

Y=45+6x  /substract 45 from both side
 we get:
y-45=45-45+6x
y-45=6x   / divide bot sides by 6
(y-45)/6=6x/6
which equals to 
(y-45)/6=6/6*x
(y-45)/6=1x or just x