PARENTHESES.

.

18. $3a(b-c)$.

19. $5a(b^2+c)$.

20. $5a(b^2-c^2)$.

21. $5a^2(b^2-c)$.

The numerical value of an algebraic expression is the number obtained by putting for the letters involved the numbers for which these letters stand, and then performing the operations required by the signs.

1. If $b=4$, find the value of $3b^2$.

2. If $a=7$, $b=2$, $c=3$, find the value of $5a{b}^2{c}^3$.

Exercise 3.

If $a=7$, $b=5$, $c=3$, find the value of