answer:
y=5x
Explanation:
use a graphing calculator. go to STAT then EDIT. in L1 put 3 and 7. in L2 put 15 and 35 (so they line up horizontally with 3 and 7). then 2ND QUIT. then STAT > CALC (4). then VARS > Y-VARS (1) (1). then enter.
y=mx+b
y= 5x
or be like me and realize at the end that i was slow and could have just (15/3) and (35/7). do the calculator thing in the future