Question

Given the point (5,6) and the slope of 5, find y when x=21

Answer 1

$\sf{Slope =\frac{y_2-y_1}{x_2-x_1}}\\\\5 = \frac{y - 6}{21-5} \\\\5=\frac{y-6}{16}\\\\16(5) = y-6\\\\80 = y-6\\\\y = 80 + 6\\\\\boxed{\bf{y=86}}$

Answer 2

If slope is 5, 5.(x-5)=y-6 so 5x-y-19=0. When x=21, y=86.