Answer:
x=y-3
20 years
Explanation:
So lets make some variables to equate them, let x be the lifespan of the tiger and y be the lifespan of a lion. Since the lifespan of a tiger if 3 years less then a lifespan of a lion , lets subtract the 3 year difference from the lifespan of a lion.
x=y-3
Lets input the known lifespan of a tiger.
17=y-3
Add 3 to both sides to get our variable on its own.
17+3=y-3+3
do the addition
20=y
The lifespan of a lion is 20 years.