Consider the utility function $U(X_1, X_2) = X_1X_2$. Assume that the consumer has budget $M=18$, that the prices are $p1 = p2 = 1$ to start, and that the price of good $1$ rises to $p1 = 2.25$. Find the loss of consumer's surplus resulting from the rise in $p1$. Round the answer to one decimal place.
I'm working through a sample question as part of my microeconomics class, I have worked out the consumers demand function for good $X_1$ is
$$X_1 = \frac{m}{2p1}$$
I don't know how to find the loss of consumer's surplus from this. Any help would with an explanation would be greatly appreciated.