Evaluate the expression when $a=-18\text{and}b=-3$.

$a+\left(-8\right)$

To add two numbers with similar signs, add their absolute values and keep the similar sign with the resulting number.

To add two numbers with opposite signs, subtract the absolute value of smaller number from absolute value of larger number and keep the sign of larger number with the resulting number.

If two positive numbers 2 and 3 are added, then the resulting sum will be $2+3=5$, which is positive as well.

And if two negative numbers $-2$ and $-3$ are added, then the resulting sum will be $\left(-2\right)+\left(-3\right)=-5$, which is negative as well.

The sum of the numbers $7-3=4$ is positive because the larger number is positive as well.

And the sum of the numbers $3-7=-4$ is negative because the larger number is negative as well.

In order to evaluate the value of the expression $a+\left(-8\right)$, substitute –18 for a in the given expression to get

$a+\left(-8\right)=-18+\left(-8\right)$

Now to add the numbers further, observe that both numbers have the same signs so the numbers will add.

So, add the numbers and put the same sign with the resulting number as shown below

$\begin{array}{c}a+\left(-8\right)=-18+\left(-8\right)\\ =-26\end{array}$

Thus, the value of the given expression is $-26$.