The expression you've given, $4x + 5y$, is a linear algebraic expression in two variables: $x$ and $y$. Since no additional information or equation is provided (such as setting the expression equal to a number), it remains an expression rather than an equation to solve. Here are some things you can do with this expression:

1. Simplify it if there are additional terms, but as it stands, it's already in its simplest form.
2. Evaluate it if specific values for $x$ and $y$ are given.
3. Graph it if it's treated as part of a linear equation, like $4x + 5y = 0$. This would represent a line on a Cartesian plane.

Do you have specific values for $x$ or $y$, or do you need help with anything else related to this expression?

Related questions:

1. How do you evaluate $4x + 5y$ for specific values of $x$ and $y$?
2. How do you convert $4x + 5y$ into an equation for graphing?
3. How can you solve $4x + 5y = 20$ for $x$ or $y$?
4. What are linear expressions, and how do they differ from quadratic ones?
5. How do you find the slope and intercepts from a linear equation?

Tip: When dealing with expressions involving two variables, it's often useful to evaluate or graph them by giving specific values to the variables.