As you can see the picture has a 90° angle, so all you have to do is subtract 42° from 90° to get the answer for "a".
Here are the steps,
a+42°= 90°
a= 90°-42° (The sign of the number changes when it goes to the other side . So when +42 goes to the other side, it becomes -42 and subtract 42 from 90)
a=48//