Its fairly straightforward. Since the bottom equation only has one unknown,x, because y=1.3, you can plug y in and solve for x. Once you find the value of x, you then have the value for two variables, x and y, and again have one unknown coefficient a. To solve for the coefficient you just plug in your y value (1.3) and your x value (which can be rounded to 0.42). Using a little bit of algebra, you can then solve for a which should be a=2.108. I am not sure if your teacher wants you to solve it this way but you could also use the elimination method or substitution method that you would of learned when discussing system of equations. But no matter which way you do it, the math follows the rules. Hope this helps. I’d suggest you solve it yourself to double check my work.
To verify my credibility,
I am a Mechanical Engineering major w/ minor in mathematics
        
             
        
        
        
Answer:
No, she has more than one independent variable.
Step-by-step explanation:
 
        
             
        
        
        
The formula for surface area is SA = 2 (wl + hl + hw). Plugging in the numbers that we know, we get SA = 2 (100 + 10h +10h). We know that the crust accounts for half of the surface area. 
The surface area was not provided so this is the closest I can go without the provided surface area. I hope this helps you :)