Because the two angles add up to equal 90, you would make an equation set to equal 90. On the other end you add the measures of your angles together because they are adding up to equal 90. This would look like 2x+5+35=90 and you solve from here. Add like terms making it 2x+40=90 then you subtract 40 from both sides making it 2x=50 and then divide by 2 on both sides which leaves you with x=25.