I hope you have been able to solve all the questions of the last exercise. In case you have not been able to solve them, here are the hints of some tricky questions
Multiply the equation by 2 in ab/2 = 3a + 15b + 5
Put x = a , y = ab, solve for x and y, then get a and b (reject non integers) in 2a2b = 5a + 2ab + 2
Similarly solve a3b2 = 3ab2 + 5a2 - 4 putting x = ab2 , y = a2 , get x and y and thus get a and b and then reject non integers.
Therefore, now you can solve equations of type pxa+cyb+d = qxayb + rxcyd + s where p,q,r,s,a,b,c,d are known integers and x and y are unkown integers. Just put m = xayb and n = xcyd , then solve pmn = mq + nr + s for unknown m and n and then get value of x and y (rejecting non-integers).