So first you make the fractions #\frac {y} {3} # and #\frac {4y} {9}# have equal denominators. You do this by multiplying the denominator and numerator by 3 for the fraction #\frac {y} {3}#. This gives you #\frac {3y} {9}#. Then you add #\frac {3y} {9}# and #\frac {4y} {9}# together to get #\frac {7y} {9}#. Finally, you substitute 7 for #y# to get #\frac {7*7} {9}#. This gives you #\frac {49} {9}# or putting into mixed fraction gives you 5#\frac {4} {9}#.