[tex]3x+15y=45[/tex]
[tex]x+5y=15[/tex] ↦ Divide the whole equation by 3.
[tex]5y=15-x[/tex] ↦ Move the x to another side.
[tex]y=\frac{15-x}{5}[/tex] ↦ Move 5 to divide 15-x
[tex]y=\frac{15}{5}-\frac{x}{5}[/tex] ↦ Separate
[tex]y=3-\frac{x}{5}[/tex] ↦ Divide 15 by 5 as we get 3.
[tex]y=-\frac{x}{5}+3[/tex] ↦ Arrange in the form of y = mx + b or the slope-intercept form.
That's the answer.
