# Multivariate Linear Regression with exponential trend-line

The dataset that I'm working with has 2 independent variables (qty, volume) and 1 dependent variable (cost). When I plot individual X with Y, it turns out qty vs cost gives an exponential decay trend line while volume vs cost gives a linear relationship.

I'm trying to come up with a linear model (since it's beginner-friendly) to predict the cost when a new input, volume set is given. I tried to excel and while the statistic output looks reasonable, the predicted values are off quite a lot. I read up on some articles and there is something regarding log transformation so that both qty and volume have a linear relationship with cost. I'm not sure if log transformation applicable in this situation. What would be the best way to approach this problem?

I plan to use R instead of excel next.

Here are some outputs of what I've done  Note: Thickness, width, and height essentially become volume so it doesn't affect the data much

$$cost = a1 * log(qty) + a_2 * volume + a_0$$