ANSWER
[tex]\begin{gathered} f=\frac{5.00-0.34p}{0.49} \\ p=\frac{5.00-0.49f}{0.34} \\ p=4.618\approx5\text{ postcard stamps} \end{gathered}[/tex]EXPLANATION
The equation that represents the situation is:
[tex]0.49f+0.34p=5.00[/tex]To solve for f, make f the subject of the formula from the equation:
[tex]\begin{gathered} 0.49f=5.00-0.34p \\ \Rightarrow f=\frac{5.00-0.34p}{0.49} \end{gathered}[/tex]To solve for p, make p the subject of the formula from the equation:
[tex]\begin{gathered} 0.34p=5.00-0.49f \\ \Rightarrow p=\frac{5.00-0.49f}{0.34} \end{gathered}[/tex]To find how many postcard stamps Noah will need if he puts 7 first-class stamps, solve for p when f is equal to 7.
That is:
[tex]\begin{gathered} p=\frac{5.00-(0.49\cdot7)}{0.34} \\ p=\frac{5.00-3.43}{0.34}=\frac{1.57}{0.34} \\ p=4.618\approx5\text{ postcard stamps} \end{gathered}[/tex]