Write the equation to a line that goes through (3,15) and (7,21)

Y=mx + c

first find 'm'

m=y2-y1/x2-x1

m= 21-15/7-3

m=6/4 or 3/2

then substitute values in from either points

y=mx+c
15=3/2 * 3 + c
15=9/2 + c
15-9/2=c
11/5=c
therefore the equation must be y=3/2 x 11.5
note: you substitute with either (3,15) or (7,21) you will get the same answer either way.