The given equation is Y1 = 0.3x + 12
First of all, note that you need one equation for each variable that you need to solve independently. For two variables, you will need two equations.
Here, the variables are Y1 and x and we have only one equation. And hence, we cannot find an independent solution to either of the variables and the solution will be interdependent.
For example, Y1 can be solved as 0.3x + 12. If the value of x is taken as 0, then Y1 = 0.3(0) + 12 = 12.
Similarly, if x is taken as 1, Y1 = 0.3(1) + 12 = 12.3
We can also find the value of x in terms of Y1 as,
0.3x = Y1 - 12
or, x = (Y1 - 12)/0.3
So, if we take Y1 as 0, x = (0 - 12)/0.3 = -12/0.3 = - 40.
For Y1 = 3, x = (3 - 12)/0.3 = -9/0.3 = -30
Thus, we need to either have a value of either x or Y1 to solve the other variable (Y1 or x). The variable will be solved in terms of the equation's other variable.
Sometimes, for this type of question, we are given the coordinates of a point through which this line passes.
Similarly, the given equation can also be considered as the equation of a line of the form y = mx + c, where m is the slope of the line and c is the intercept.
If that were the case, the slope here is 0.3 and the intercept is 12.
If we know a value of x that satisfies this equation, that is, the x-coordinate of a point through which this line passes, we can calculate the value of Y1.
Hope this helps.