Exercise 15 Page 85

To solve the equation we have to isolate a on one side. By adding 5 to both sides, we can cancel out the - 5 and isolate a.

-5+a=21

AddEqn

LHS+5=RHS+5

-5+a=21

RearrangeEqn

Rearrange equation

a+(-5)+5=21+5

AddNeg

a+(- b)=a-b

a-5+5=21+5

AddTerms

Add terms

a=26

Therefore, a=26 is the solution to the equation. To check that we have the correct solution, we can substitute a=26 into the original equation and simplify.

-5+a=21

Substitute

a= 26

-5+ 26? =21

RearrangeEqn

Rearrange equation

26+(-5)? =21

AddNeg

a+(- b)=a-b

26-5? =21

SubTerms

Subtract terms

21=21

Since the left-hand side and right-hand side are equal, a=26 is the correct solution.