The ln function is the same as loge. So you can rewrite the equation as loge(16) = -2x. Then the logarithmic function and exponential will have the same base and the answer to the logarithmic will become the exponent of the exponential.
P(x) = 1600+1200*x, clearly x represents the number of cars solved as it's a variable. 1600 is the base pay and 1200 is the commission he get on each variable/car