determining the next random number pseudorandom number generator?

Question

I have given 3 numbers let's say basic example x_0=5, x_1=6 and x_2=2 and modulus p is 7,

x_n=(ax_n-1+c)modp where p=7, I am trying to generate linear equations to solve but I am kind of lost

I do this

6=(a5+c)mod7 therefore 7*x+7=5a+c 
2=(6a+c)mod7 therefore 7*y+2=6a+c  

now I can subtract them and have 3 unknown x,y and a but two equations to solve,

I do not understand how to generate two equations with two unknows

I really appreciate your help

I read this but I could not understand

Number of samples to predict the next number in a pseudorandom number generator