2. Use the second equation (for y) to substitute into the first.
.. x +(2x +4) = 7
.. 3x +4 = 7 . . . . . collect terms
I know you can solve it from there. (x, y) = (1, 6)
3. Let "o" and "r" represent the numbers of orange fish and red fish, respectively.
.. o +r = 31 . . . . . there are 31 fish in a tank
.. o = 7 +r/2 . . . . there are 7 more orange fish than half the number of red fish
The second equation can be substituted into the first.
.. (7 +r/2) +r = 31
.. (3r/2) = 24 . . . . . . subtract 7
.. r = (2/3)*24 . . . . . .multiply by the inverse of the coefficient of r
I know you can solve it from there. (o, r) = (15, 16)