If you want to see where n=1.5 comes from, draw two straight line graphs: y=6n+18 and y=42-10n on the same graph. To draw the first graph mark the y intercept on the y (vertical) axis (y=18, when n=0) and mark the n intercept (n=-3, when y=0) on the n (horizontal) axis. Draw a line through these two points to get y=6n+18. For the other graph, 42 is the y intercept and 4.2 is the n intercept. Draw a line through these intercepts. The lines cross where n=1.5 (y=27).