we will use the eqaution
[tex]c-c_1_{}=(\frac{c_2-c_1}{p_2_{}-p_1})(p-p_1)[/tex]c1= 675
c2=525
p1=3
p2=5
Substituting into the formular yields
[tex]\begin{gathered} c-525\text{ = (}\frac{675-525}{5-3})(p-3) \\ c\text{ -525 =75(p-3)} \\ c=\text{ 75p -}225+525 \\ c\text{ =75p+300} \\ \text{Two phones will cost} \\ c\text{ = 75 x 2+300} \\ c=\text{ 450} \end{gathered}[/tex]