By default, u are supposed to make the eqn at the right = 1 for integration. But in this case, since k is a constant, to make integration easier, u just need to bring over (100 - w) to the left side.

After forming the eqn, directly integrate. Use recognisable forms to integrate left side. Right side integrate k dt is just k + c.

After that, manipulate the eqn to make w the subject. However, u cannot leave the equation alone due to presence of c that acts a constant. (qn doesn't need you to find k)

Hence, when time equals 0, no words can be learnt. Hence, W = 0. From there, find what is c and sub back into the prev equation to form W=A - Be^-kt